rolling vibration n prop shaft questions

[The Loves]

I have a problem with a rolling speed vibration that occurs while under
acceleration (any gear) up to approx 40 mph and then disappears.
Wheels are balanced and round, tires new and cv joints check out fine but
when inspecting the prop shaft, the u joints and center support bearing seem
fine but the connection at the transmission has a lot of slop to it. Up and
down, forward and backwards. Could this be the culprit and if so is it
serviceable without having to buy a new complete prop shaft? Maybe used in
the Wa. State area?
The car is a Syncro wagon and I thought I would post it over here in
addition to the syncro group sine I know these are the same parts as on my
4kq
TIA for any insight here.