Could be as easy as just needing new gaskets for the universal ball and collar. I would have a u joint kit too, just in case you find your vibration in the in the form of a u-joint.
Remember there is a seal at the end of the ball collar, that is retained around the end of the torque tube, make sure you have that seal in place and there are the two flat washers -one either side of the cork seal, if not it will be a leaker. I have seen a few trucks with zero or just one flat washer in them before.


1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
In the DITY Gallery
Video of the 261 running

1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration