I have a 1972 chevelle SS with a 402 and 4 speed,12 bolt 3:31 gears. I notice that I have a small vibration, more noticable at speeds over 50( i can still feel it at lower speeds). I can feel it coming threw the hurst shifter.YOu can also feel it in the car as well. The car has new tires, motor mounts and wheels,u joints ,bushings in trans ect. it is diffently in the rear of car. I balanced the drive shaft trying to rule out that possibilty,but it was fine. Here is my first Question I put 3" drop springs in car. could it be that the pinion angle is wrong? The vibration is not real bad but very noticable. And i dont recall it being in there before. I didn't drive it that much before restoration. thanks

Could be a pinion angle issue, but they are usually torque sensitive or cause a launch shudder.

Is the rear U-joint properly positioned between the tabs on the differential yoke?

Yes it is centered.

You might try shiming the transmount to raise the rear of the transmission. This will change the +- relationship of the pinion angle. If it gets worse or better, then the adjustable rear uppers are in order. Like '36' said, normally pinion angle problems are torque sensitive. You will either be on or off the gas and feel it. And running along at a constant speed the vibration goes away.

Try Checking the rear trans mount It's probably broken

Mount is good> thanks for all your replys.Ill keep check=ing in to it.