Have you checked/changed the carrier bearing? That's the bearing that supports the back of the front shaft. They are usually mounted in rubber to dampen vibration and the rubber can wearout, stretch and crack. Also the bearing itself can go bad. This is a common source of vibration in two piece setups that have a lot of miles.

Also you mentioned you had the tires balanced. hopefully they were also checked for out of round or ply seperations. I heard of several instances just recently where mysterious vibrations ended up being tire problems.

Good luck with it

Pat