I'm saying that there's enough preload on the clutches that it should take 120-180 ft/lbs to make them slip if your trying to rotate one wheel while the other is held stationary. Even if it's worn out according to the FSM (<30 ft/lbs) that's still enough that if both wheels are off the ground and you turn one, the other will turn with it in the same direction.

Now, if you're going around a corner while coasting, the preload is the only resistance to the wheels going different speeds and 150 ft/lbs wouldn't make you drag a tire on a 4500lb truck so it's not likely to be noticeable when easing around. However, when you're on the gas around a corner you're applying a rotating force to the pinion shaft, which transfers it to the ring gear connected to one axle then through the pinion gears/side gearsto the other axle, etc. Under load, when the pinion gears try to transfer loadfrom oneside gear to the other side gear while turning, the gears try to ride up which causes a thrust force to push the side gears apart and compressingthe clutch plates on either side, which increases the amount of torque required to make them slip. That's how a "limited slip" limits slip. It works the same ifone wheel is on driy and the other on a wet surface. Try this site for more info; http://auto.howstuffworks.com/differential.htm

 

i think corp. rear LD means light duty as opposed to larger commercial trucks.