Well my car has vibrated since I've had it. I've tried new bearings, new pads and rotors, I've had two sets of rims and tires. Recently I installed a LS1 driveshaft it did help it but its still there. Anyway while installing the driveshaft I found that the pinion yoke had some play(pushing up on driveshaft). Should it have play? I think this could be my problem.

 

Do you you have a poly transmission mount on the car?

 

No it has stock style transmission mount.

 

I would check out the pinion yoke and make sure its good. If it is then I would first try to rotate the DS 180 degrees and see what that does. If that doesn,t help and the pinion turns out fine then you may have a cracked flexplate.
If you get in there and the flexplate is good I would then rotate the converter one slot around and see if that cures it.
Mine vibrated when I had the 60E in it but it was because of the poly mount, one day the vibration went away and I was like thats weird so I checked it and my tranny mount had come loose.