First Chevy Ive owned, just purchased a '96 C2500, 5.7, 14 bolt, 8 lug, semi-float G80. The guy I bought it from said it had a brake grabbing but I dont think it is the brakes. It has a thumping sound especially at higher speeds. I jacked it up and ran it up to 60 mph but no thumping sound. I checked the mid-shaft bearing, looks good, u joints look good best I can tell. One thing I did notice is the drive shaft seems not to be perfectly in line between transmission and rear end, a slight degree of angle (from left to right, not up and down). I dont know if this is normal, or? I would think that if this situation was a factor which is causing the thumping sound, it would sound off while on the jack stands. So, before I send her to the mechanic, I decided to open up the differential and see what it looks like, and change the gear oil....trying to figure out which oil to use was so confusing...didnt know whether to scratch my teeth or brush my ass, but have finally decided on Valvoline High Performance 80w/90 or MasterPro 80w/90...both have no LS additive...probably go with the Valvoline. I have doubts that this will remedy the thumping sound, but at least I will have fresh gear oil.