I would open the "bell/ball" and inspect the u-joint (as described by buoymaker, I think).
Replace u-joint and bushings, if necessary.
Replace the rear rubber seal that slides on the thoroughly cleaned-up outside of the torque-tube.
Replace and "adjust" the bell/ball gaskets (as described in the Shop Manual).
There is no need to drop the transmission to do this.
No need to cut/glue any seals /gaskets.
If you can get the vehicle high enough to easily scoot in-and-out on a "scooter board" this could be a 2-3 hour job.
The external parts clean-up takes the most time - it has to be done in either "technique/procedure".
