I had this issue with too much pinion angle. The car was smooth going down the track but when I let off the gas I was getting a vibration. The pinion would fall on decel. and put the rear joint in a bind. Too many passes before I could get it fixed caused the tail housing to crack by the speedo hole. I ended up winning that day but was dripping trans. fluid on the last pass! I was a young kid and had set the pinion angle up incorrectly. Lesson learned! Kevin