MiroBG,
Sounds as though the universal joint could be worn.... lift the rear end of the 'tang and get the tires off the ground (NOW is a good time for jack stands) and grab hold of the drive shaft try to rotate it. You're looking for excessive movement of the joints front(
tranny)and rear(hoghead). While you're at it try to move the tail shaft of the
tranny and turn the rear wheels by hand listening and feeling for play in the differential.
This should give you a starting point and if I have missed anything PLEASE somebody correct me or add to this.