You could check the drive shaft Carl. I always put the shaft all the way in the trans bushing and like to have no more tan 3/4 inch back to the rear end yoke. Too much slack will give you a vibration.
Is the rear tranny mount OK? Also do you have solid mounts or rubber? I like to use solid mounts with rubber trans mount. It does give you more vibrations but they won't break. Just my thoughts.