I had been reading the threads on driveline vibration.
I can't seem to find a comment on tailshaft endplay.

Any idea as to how much "endplay" is considered
acceptable ?
I have about .160 inch up and down "play"
(thumb pressure)

My trouble is a vibe at around 2900 rpm in fourth.

Unfortunately I changed too much all at once ...

10 bolt, Zexel Torsen posi, 3.73 Richmonds, LS1 aluminum
drive shaft, rear discs, springs, shocks, bushings, control arms

The wheels have been balanced, the front end aligned...

 

Also I checked pinion angles.
Front and rear at about 1 degree
(Using a cheapie angle finder from Sears)