Spool
Overview: A spool is just a solid chunk of metal that connects your axleshafts together permanently and forever. A mini-spool drops inside your factory carrier and replaces the spider gears. A full spool replaces your existing carrier and is what you should run, if possible. Don't run a spool in the front 'cause you aint a gonna be able to turn; they're for rear applications only.
Street:**
A spool is noticeable on the street when cornering sharply, but is very predictable, and we actually prefer one in the rear of a short-wheelbase vehicle over an automatic locker. Since there's never any loading or unloading, you don't get the jerking or banging associated with automatic lockers. It will increase your turning radius somewhat, and you'll notice your rear tires will scrub and chirp around corners.
Off-road:*****
Since a spool never unlocks, it's extremely predictable off-road. And since it's a solid chunk of metal, we've never been able to harm one. We haven't really found the loss of turning radius to be a hindrance. That's what Reverse is for. We hypothesize that a spool is actually easier on axleshafts off-road because there's never any sudden loading or unloading. Power is always evenly distributed to both shafts, and traction is generally phenomenal.
Good: Inexpensive, simple, lightweight, and strong.
Bad: Tires will chirp when turning on pavement, tire wear is increased, and the turning radius increased. Really only for a rear-axle application that's primarily used off-road.
OX Locker
Overview: An OX locker is a cable-operated locker. When disengaged, it's an open differential. When it's locked, it's a spool. The cases are burly, and the diff covers are heavy steel built to withstand rock hits and prevent deflection of the shift mechanism.
Street:*****
Since it's an open diff when unlocked, it's unnoticeable on the street. Make sure the cable is adjusted properly, or you may sense some ratcheting as the locking toothed gear attempts to drop into place.
Off-road:*****Works well as a front or rear diff to help keep axle parts alive and allow you to turn sharper. Older non-spring-loaded shifters can be difficult to engage in tight situations, but most of those have been swapped out and are off the market by now. The new version of the shifter is much better and easier to use.
Good: It's extremely strong and is an open diff when you want it or a spool when you want it. Great for a daily driver or hard-core abused Jeep. Perfect for front and rear applications. Comes with a bombproof differential cover, and the beefy locking spool design will survive a catastrophic shaft or U-joint failure.
Bad: The cable must be adjusted properly, or the unit will not function correctly, so make sure it's installed by somebody whose primary diet isn't bananas and peanuts from the zoo. Early shifter can be difficult to engage. The heavy industrial cable coming out of the cover can be vulnerable to trail debris in some applications. Also, the heavy cable needs to be routed into the cab, and the shifter is somewhat inconvenient to try to locate cleanly in a Wrangler or other vehicle with a small interior.
TeraFlex T-Locker
Overview: The T-locker is a pneumatic locker that uses a relatively low pressure to activate it. Unlike an ARB, which uses electric solenoids, the T-Locker uses an industrial pneumatic switch that either sends pressure to the locker (engaging it) or releases pressure (disengaging it). All the air doohickies are kept outside of the diff cover, so there are no internal seals to fail.
Street:*****
Like most selectable lockers, the T-locker made itself invisible on the street. When disengaged, the unit functions as an open diff, so all handling remains as stock.
Off-road:****Really, the only thing taking away a point from the T-locker was its inability to engage and disengage crisply. When locked, it functions like a full spool, but we found the need to jockey the vehicle around. Snaking S-turns and throttle application to spin tires was sometimes needed to get the locker to engage or disengage.
Good: In our opinion, it's the simplest pneumatic locker out there that shouldn't become susceptible to seal failure. Really thick and strong diff cover, and the locker itself is a beefy chunk of metal. Axleshaft failures shouldn't prove to be a problem for this locker.
Bad: Tightness of the machining didn't allow the locking pins to easily engage and disengage the locker. It'll probably get better as the unit ages. We'd like to see quicker engagement.