I just had 3.31 gears put in and the car has a vibration when letting off the throttle. My mechanic thinks it's the driveshaft. He has put some clamps on it to try and balance it but can't get the vibration to fully go away. It only does it when letting off at 80 now. I'm sure it does it higher than that but he won't drive my car any faster, which I completely respect. He said I either need to take it to a speed shop to get it properly balanced or get a new driveshaft. I've read that manual's have a vibration when letting off the gas but mine's an auto. Is this normal or do I need a new driveshaft?