Types Of Differentials
First off, we have the open differential, as discussed in our opening paragraph. This is essentially an iron or steel case with two (sometimes four) pinion gears, two side gears, and of course, the ring and pinion gearset. This sounds great in theory, and probably works quite well on a car with low performance on a perfect section of road. But start putting some power to the rear wheels or drive through less than perfect traction situations like wet roads, snow, or mud, and you get the "one-wheel-peel." Why? It's quite simple really. When you have an open differential and one wheel has the smallest amount of traction loss (slippage), the open differential sends all of the incoming power to the slipping wheel because it has no way of enacting force on the non-slipping wheel. This prevents the car from ever getting or regaining traction. Adding a limited slip, locker, or spool, will solve this problem and each one has its specific way of solving said problem.
A limited-slip style of differential is the most commonly known upgrade that combats the effects of tire spin with an open differential. Ford has called its version the Equa-Lok, and later the Traction-Lok. Both use a combination of alternating steel and friction clutch plates, like in an automatic transmission, under spring tension. The spring helps "lock" the axles together for traction to both wheels by exerting pressure on the clutch discs, but the alternating clutch plates allow sufficient slippage during cornering without noise. This is the system most of us are familiar with, but there are other limited-slip offerings in the aftermarket, many of which use a similar series of clutches and/or springs to provide optimum power to both wheels, while allowing slippage for cornering. The true downfall of a limited slip is the wear induced by the clutches (though a limited slip is usually able to be rebuilt with a "clutch kit") and the fact that, while the traction is split to both rear wheels, slippage can still occur, especially as the clutches wear or the spring(s) loses tension. You can actually overheat a limited slip and burn the clutches and ruin the spring(s). Clutch-based limited slips also require a friction additive to the gear oil, though many gear oils today have the additive properties built in. It's something to remember if your Ford has a limited-slip differential installed.
Next up we have the lockers, with the Detroit Locker being one of the more common one Fordophiles may have heard bandied about in car magazines and web forums. A locker, for lack of a better description, locks the two rear wheels together, but still allows for differentiation in turns. The differentiation occurs when the inside wheel and outside wheel speeds are different and the axles overcome a preload spring (or springs) and allow the locker to unlock. A locker does direct engine power to the wheel with the most traction, unlike an open or limited-slip style of differential, but lockers are notoriously noisy and that can turn some people away. They have a tendency to ratchet or "click" while turning and sometimes their disengagement to allow differentiation at low speeds sounds like you're breaking parts back there!