Thanks for the feedback guys. Although no one can make a definative diagnosis based on the limited info, the group consensus (drive shaft) is a great help to me as I had assumed from getting under there and twisting on either side of the joints that I could rule this favorite suspect out. Pointing me to something this simple has at least cheered me up.

Also, I infer that the driveshaft has to be under load to cause this vibration, since it does not vibrate under free-wheel conditions. This makes some sense as without some load it might not stress the u-joints into an out of center condition.

I will also try to take diagonal measurements from the axel to the front end to see it the rear end has gotten sideways.

Regards, Leroy