My gut says that if the motor mounts were tight to the engine block, this is not your problem. The weight of the engine would negate any vibration. But the mounts and tight bolts need to be addressed of course.
You say you just put an AOD in the car. Look at everything around the AOD for a "sound short". Metal to metal contact. The AOD is a bit larger and the cross member was replaced. I found an aftermarket conversion cross member that was designed in such a way that it touched the tail shaft housing on the automatic. Caused a similar the faster you went the more you felt vibration.
The other thing is the u joints and drive shaft as mentioned. A vibration that gets worse as the drive shaft spins faster is a sign of driveshaft balance problems. Unbalance tires seem to cause a thumping so to speak and not a severe vibration.
Try the mounts but come on back if you need "mo hep".
Rob
