Well...posi units work like brakes in a way...as you accelerate the pads in the unit apply pressure to both sides making it a full locker...if those pads are worn or broken the wheels will spin off the ground, but under load nothing will happen. So becareful with yer unit man....my dad got one for our 77 c20 and it was something like $500 for it....also it could be your rack gear as well.
