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...