Most drive vibrations are caused by mismatched angles on the drive shaft. When you are under power the nose of the diff rises changing the angle. Look for movement in the U bolts. If you put a spanner on the nuts they may feel tight when they are not, clean the nuts and mark the position. if you loosen the nuts one at a time and then re tighten them you will know they are tight. Can you fit a camera under the van aimed at the pinion. Eric.
Does it change with the gear at all? On/off throttle typically points to a rear driveshaft angle issue. When you load/unload the rear axle, the pinion moves up/down a degree or two. Its fairly easy to use a angle finder to double check the angles at the flanges.
If the vibration proportional to speed, or does it stay the same frequency?
Thought I would provide an update. So the transmission mount had that tally collapsed as well as the the universal joints. Replaced the mount and had the tail shaft rebuilt inclouding a new centre bearing. All up about $1500. Runs beautifullt. Now. Off camping in it for the weekend.