I have a 94 z.auto. The rear end has 3:73's and an eaton posi and diff girdle... I thought maybe the drive shaft was 180 out so I spun it around tonight and that didn't help.. anyways here goes... after 75 mph I start to get a slight vibration.. if I let off its even worse.. The Car is an auto.. and this happens in overdrive obviously... but I have no clue what to do next.. u joints??

 

Pretty typical problem with these cars and the stock steel driveshaft, unfortunately . Mine's pretty noticable too ... I've got thoughts of bringing it to a driveline shop and seeing if they can balance it (might help? ) ... otherwise most people say that aluminum shafts cause hardly any vibrations at all (you might want to consider finding a used LS1 aluminum shaft, or a 1LE aluminum DS) ... I can't remember if LT1's ever came with aluminum shafts, unless they got the 1LE treatment??

Are you running a poly trans mount??

 

No poly mounts at all... and I need to be... gonna switch out the motor and trans mounts after christmas

 

mine does it as well, but only on deceleration from 90 or so