Have you run the car in gear with the wheels up and watched for eccentric rotation of the drive axles? Make sure they are rotating smoothly, with no oscillation. Also check the same things on the driveshaft. Like auditech79 said in a post awhile back, vibration is caused by rotating parts. I hate to say it but if you don't see any issues with these parts, you may need to think internals.

 

He says it vibrates when he does it at a stand still, so it has to be engine related.