Open vs Limited Slip vs Locker
Which Differential to Buy

Every differential is a compromise between traction and drivability. The more a diff forces both wheels to turn together, the better it climbs and the worse it behaves in a parking lot. Pick the wrong one and you either spin a tire on a wet boat ramp or fight a truck that chirps and pushes in every turn. Here is how the five common types actually behave, and which one we would put in each kind of truck.

Open Differential

An open diff uses a set of spider gears that let the two axles turn at different speeds with no resistance. That is perfect on dry pavement. The catch is that it always sends equal torque to both sides, and the torque is capped by whichever tire has the least grip. One wheel on ice or in the air, and that wheel spins while the other one sits still.

Open diffs are quiet, cheap, need nothing special in the oil and never affect steering. On a front axle of a daily driven 4x4 that rarely leaves the road, open is still a sensible choice. Brake-based traction control on newer trucks covers some of the weakness by braking the spinning wheel, but it gets hot and gives up fast in deep snow or on rocks.

Clutch-Type Limited Slip (Posi, Trac-Lok, Traction-Lok)

This is the most common factory upgrade. Spring-loaded clutch packs sit behind the side gears and resist the axles turning at different speeds. Ford calls it Traction-Lok, Dana/Spicer calls it Trac-Lok, and most people just say posi. Auburn units use cones instead of plates, but the idea is the same.

On the street a healthy clutch limited slip is invisible. It biases torque to the tire with grip until the spinning wheel overcomes the clutch preload. It helps a lot on wet pavement and packed snow, less on rocks where one tire is fully unloaded.

The weak point is the clutches. They need gear oil with friction modifier. Many synthetic limited slip gear oils already contain it; with plain GL-5 you add it, and Ford 8.8 axles take about 4 ounces. When the modifier wears out, you get the classic chatter or shudder in slow, tight turns. Our Traction-Lok chatter fix guide walks through it. If fresh fluid does not cure it, the plates are worn, and a kit like the Ford 8.8 Traction-Lok clutch rebuild kit ($158.99) puts it back to spec. Expect the clutches to fade over 60,000-100,000 miles of hard use.

For an upgrade, the Yukon Dura Grip for Ford 8.8 31-spline ($901.89) uses composite clutches and heavier preload than the factory unit. GM 8.5-inch owners can look at the Eaton Posi for 8.5-inch 30-spline ($750.99), and Jeep and Ford Dana 30 fronts have a budget option in the Dana 30 front limited slip, 27-spline ($287.99).

Gear-Type Torque Sensing (Eaton Truetrac)

The Truetrac uses helical pinion gears in pockets in the case. When one wheel starts to slip, the gears thrust against the case and transfer torque to the other side. There are no clutches, nothing to wear out and no friction modifier. Regular GL-5 gear oil is all it wants.

It is the best street limited slip you can buy. It is smooth, silent and has almost no effect on handling, which also makes it the friendliest choice for a front axle. The one real limitation: it multiplies the torque the low-traction wheel can take, so a wheel completely in the air gets near zero, and so does the other side. A light touch on the brake pedal while you stay in the throttle loads the spinning side and gets it pulling again.

We carry Truetracs for most common axles, including the 2015-2023 F-150 rear 34-spline ($803.99), the GM 1500 9.5 and 9.76-inch 33-spline ($830.99) and the Ram 9.25-inch 31-spline ($729.99).

Automatic Locker (Detroit Locker)

A Detroit Locker is locked all the time under power. Both axles turn together until one wheel needs to go faster than the other, such as the outside wheel in a turn off-throttle. Then a set of spring-loaded dog clutches ratchets that side free. That ratcheting is the click and occasional bang you hear in parking lots.

It is nearly unbreakable and gives 100 percent traction to both wheels whenever you are on the gas. The trade-offs are real. The truck pushes straight in turns if you stay on the throttle, short-wheelbase rigs can step sideways when they lock and unlock, rear tires wear faster, and on ice a locked rear can swing the back end out. It is a great rear choice for a dedicated trail rig or a heavy truck that lives in mud. We do not recommend one for the front of anything that sees regular street miles.

The Detroit Locker for 35-spline Dana axles, 4.56 and up ($992.99) is a common pick for built rear axles.

Selectable Locker (Eaton ELocker, Air Locker)

A selectable locker is an open diff until you flip a switch. Then it becomes a spool and both wheels turn together no matter what. The Eaton ELocker uses an electromagnet on 12 volts; air lockers use a small onboard compressor. Factory lockers on the Wrangler Rubicon and Power Wagon work the same way.

This is the best of both worlds and the right answer for a front axle on a serious trail truck. You get open-diff street manners and full lock when you need it. The costs are price, wiring or air lines, and one more thing that can fail. Lock it only at low speed on loose surfaces, and unlock it before pavement. If your factory JL locker will not engage, start with our JL locker actuator guide.

For Jeep JL and JT Dana 44 axles, the Eaton ELocker for M210 and M220, 32-spline ($1,554.99) is a direct fit.

Comparison Table

Add $400-$800 in shop labor for any carrier swap. The carrier comes out, backlash and carrier bearing preload get reset, and most shops will want new carrier bearings. Carriers are often split by gear ratio, so confirm your ratio and axle spline count before ordering. If you are changing gears at the same time, do both in one job; see our regear guide and the backlash and pinion depth setup guide.

Front vs Rear

The front axle steers, so anything that locks the front wheels together makes the truck resist turning. Up front, stay with open, a Truetrac or a selectable locker. Clutch limited slips work in the front but tend to make steering heavy in 4WD. Out back you have more freedom, and any of the five types can make sense depending on use.

Which One Should You Buy

• Daily driver or half-ton that tows a camper: Truetrac in the rear. No maintenance, no surprises.
• Heavy towing or plow truck: clutch limited slip or Truetrac in the rear, open or Truetrac in the front. Skip automatic lockers; abrupt lock-up on a snowy grade with a plow or trailer is how trucks get turned sideways.
• Weekend trail truck that is still driven to work: selectable locker in the rear, Truetrac or selectable in the front.
• Dedicated rock or mud rig: Detroit Locker in the rear, selectable locker in the front.
• Muscle car or street performance: Truetrac or a clutch limited slip with fresh modifier.

Parts for the Swap

Gear oil and additives are in our fluids section. If the carrier is coming out anyway, the master overhaul kit buyer's guide explains which bearing and seal kits to replace at the same time. For transmission and transfer case parts further up the driveline, CoreTransmissionParts.com is our sister source.

FAQ

Does an Eaton Truetrac need friction modifier?

No. A Truetrac uses helical gears, not clutch packs, so there is nothing for a friction modifier to act on. Fill it with the regular GL-5 gear oil your axle calls for. Adding modifier will not hurt it, but it does nothing.

Why does my limited slip chatter or bang in tight turns?

On a clutch-type limited slip, chatter in parking-lot turns almost always means the friction modifier is worn out or the gear oil is the wrong type. Drain it, refill with the correct gear oil plus the specified modifier (Ford 8.8 Traction-Lok takes about 4 ounces), and drive some slow figure eights. If it still chatters, the clutch plates are worn and need a rebuild kit.

Can I put a Detroit Locker in the front axle of a daily driver?

You can, but you will not like it. An automatic locker in the front fights the steering, makes the truck push in turns and is unpredictable on ice. For a front axle that sees street miles, a selectable locker such as an ELocker or air locker, or a Truetrac, is the better choice.

What is the best differential for towing and plowing?

For a rear axle on a tow or plow truck, a clutch-type limited slip or a Truetrac is the safest pick. Both give you traction on wet ramps and packed snow without the abrupt lock-up of an automatic locker, which can kick the rear end sideways on ice under load.

How much does it cost to swap in a limited slip or locker?

The carrier itself runs about $450-$900 for most clutch limited slips and Truetracs, $700-$1,100 for a Detroit Locker and $900-$1,600 for a selectable locker. Shop labor to install it and reset backlash and carrier bearing preload usually adds $400-$800, plus new carrier bearings and gear oil.