Well i when i was putting my car back together, i used a driveshaft from a '95 Z28 b/c it was painted and nice and my old one was rusty. So before i put it in i did the u joints on it. I soon found there was a very very bad vibration at 80mph..especially on deceleration. I soon switched it for an aluminum one my friend had and the vibration was gone completely...all the way up to 125mph. I then started to notice more and more as time went by that the vibration was coming back. Soon it was so bad that it would be bad at 45 mph deceleration. So i did the u joints on the aluminum ds and found that the front one was totally mangled...ok i thought..problem solved. Well now i have a vibration after letting off at 90mph..not nearly as bad as the first time with the 95 ds...but its still pretty bad. My rear tires have been balanced as well. So im out of ideas...can bad u joints throw a driveshaft out of balance? That seems to be the only option left to me..