I have an 87 Iroc-Z with a 305 Tpi, 5 spd man trans, and a borg warner rear end with four wheel disc brakes. At any speed above 65 there is a vibration from the back half of the car that gets worse with speed. So far I have replaced the U-joints, had the driveshaft straightened and balanced, replaced all the bearings in the rear axle, and transmission, and one wheel. None of these things made a difference. I have run the car on the hoist with the wheels removed and also shut the engine off at these speeds and it still vibrates which helps eliminate some possibilities. Any information to help fix the problem would be appreciated, at this point it is getting personal. Thanks Adam

 

Adam,
You seem to have done just about everything. The only things I can come up are: the rear rotors could be out of balance, or the pinion angle and the transmission tailshaft angle are not equal when the car is at rest. If you can find a place that still balaces the wheels while they are on the car, you can try rebalancing the rear wheels. The pinion & tailshaft angles are tough to gauge, unless they are way off.

 

Just another taught. You may want to check the rear wheels for side to side runout with a dial indicator. Also a possibility the there is a hard spot on one of the tires. If there is no amount of balancing will help because the tire will take a little bounce wheneve the hard spot touches the road. I assume the wheels were balanced. Has the driveshaft been modified? Are the front and back driveshaft ears perpendicular to each other (facing 90 degree apart from each other) You can also check for driveshaft runout with a dial indicator at both ends and in the center, althoug you said it was straightned out.