Conrad,
If you want to drain the u-joint enclosure, you will have to re-seal it. If gaskets break, that might turn into a more complicated procedure. If the enclosure does not leak (much), you might want to let it be (unless you are hearing noises from inside the enclosure).
The transmission fluid slowly migrates rearward into the u-joint enclosure (and visa-versa).
Fill the transmission to the appropriate level; let it sit a few hours; check and top up to fill level; check and repeat until no change in level.
