Good when on pedal in 6th gear. When decelerating, huge vibration. Thoughts? Trying to help a friend.

U-joints checked?

Jim, does it only do this in 6th? That would indicate an internal trans problem, but that's a long-shot. The ZF units are pretty bullet proof normally. They do have a LOT of gear rattle, due to the size of the gears, and the clearance needed for them. Can't say that I've ever heard of one that would create a vibration only in one gear, tho...
Otherwise, I'd agree with the u-joints being the issue, either on the driveshaft itself, or one of the halfshafts. Might check the torque on the halfshaft nut in the center of the rear hubs. That can cause some wierd vibs.

Only in 6th...and only coming off the gas

Sounds like time for tranny werk...
Before digging any deeper, tho, I would first pull the driveshaft, and check the yoke, and the tailhousing bushing, to see if there was any major wear....