Driveline Vibration

Jack there are a couple of things you can try to determine if the vibration is in the driveshaft or not. The first thing I would do is remove the shaft at the rear end, turn it 180*, test drive and see if the vibration frequency/speed/dynamics change any compared to now.
Another old timer trick is to put a couple of hose clamps on the driveshaft and keep moving the heads of the clamps around to see if the vibration changes or not. Start with the clamp heads on opposite sides of the driveshaft, then progressively move then toward each other to see if there's any change. Cheap man's balancing act, so to speak. :) Good luck.