Over the years I think I've done just about all the above; motor and tranny mounts (several), flex discs (several), shimming the center bearing support, subframe bushings, diff mount, ad nauseum.

One thing that is interesting; it absolutely does not do it until the car is fully warmed up. For the first 5-10 miles, it doesn't have a hint of the vibration, even under pretty hard throttle. But, after the entire driveline gets good and warm, it begind to vibrate under acceleration, and seems to get a bit worse the hotter everything gets. I thought masybe an expanding driveshaft was binding a bit, but after lubing and torqueing my center DS slip joint, it moves forward/rear quite easily, even with the big nut properly torqued - so can't be that.

DG