Ah okay, I understand. I linked vibration and idle together. But you will be surprised how stead and smooth these cars can idle IF any of the ignition components were naffed. Anyways, I think you're on the right track with the tranny/tranny mounts then, you probably didn't need me to tell you.
There was one thread I was reading through and the owner who seemed to have a similar issue asked if a bad torque converter could cause this. There was no responce but I got to thinking that it might be possible if the clutch packs (or whatever the system is called inside a T.C.) are perhaps not...erm...idling properly??? Does that make sense? Or maybe a bearing has worn and the torque converter is wriggling enough to cause a vibration. Just throwing a few ideas for you to mull over. I've personally haven't really heard of this issue before as generally it's usually related to idle.