Classic/Antique Car Repair: 1970 Oldsmobile Cutlass vibration, oldsmobile cutlass, 1970 oldsmobile cutlass

Question
I have a 1970 Oldsmobile Cutlass "S' with a 350 engine and turbo 350, I recently has the 2:56 differential pulled out and a used 3:08 posi dropped in and a bad vibration to go with it.  I have had the engine mounts replaced, the transmission mount checked, new U-joints put it, tires balanced and rotated, axles, bearings and the gears checked and the vibration is still here.  The vibration started immediatly after the new differential was put it.  The differential was out of a 1968 Olds 442.  My mechanic is at his wits in.  Do I need to balance the dirve shaft?  Also the drums in the rear had weights on them and the replacement drums did not, is there a difference?  Also the vibration feels like a rotational vibration.  Please help James

Answer
Raise rear of car stably off ground. Remove rear wheels and PUT LUG NUTS BACK ON. Then crank car and run in drive. You should be able to visually locate vibration. Could be bent rear axle, hub, or yoke. I would not "repair" anything alse until I located the problem. If it started with rearend there a 95% chance that is where the problem lies and not every thing else you are replacing.