You're talking about a vehicle with wheel speed sensors. The trucks have one speed sensor in the diff housing that works basically like a wheel speed sensor except it reads a toothed ring around the carrier.

 

That is probably why the rear ABS stinks so bad since it's reading off the "differential". Not a smart idea.

 

Well, it's the same thing as a wheel speed sensor basically, it tells you how fast the axle/axles being powered is/are spinning. What make it suck is the ancient technology they use, the modules and valves aren't great and it only controls the rear brakes anyway, which with a truck is meant most to keep the rear end from kicking to the side when you stomp the brakes. It works ok when it's new but the rear sensors go bad all the time. I'm 99% sure the module is bad on mine so I'm gonna take the ABS dash bulb out and voila, fixed.