There should not be any movement with the driveshaft installed in the car , no up or down and no side to side movement, EVER tsk It should rotate smoothly in a straight plane, no wobbling tsk
Wobbling(up or down or side to side) makes vibrations, straight and true do not work
It sounds to me that maybe the front U joint tranny yoke is junk scope It should not move up or down or side to side when installed on the tranny output shaft tsk
Get that good looking rascal up in the air on jack stands so you can run it in gear to find the cause twocents

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)