There should be a lip seal on the transmission mainshaft rear bearing carrier. At the rear end of the mainshaft is the input gear for the transfer case, which is held in place by a large nylock nut. The seal rides on the input gear. There should also be a paper gasket between the transmission and the transfer case that completely seals the two and prevents fluid swapping. I just had my CJ5's T150 and Dana 20 apart partly for this reason and found that the nut holding the t-case input gear was loose, allowing the gear to move and the seal to leak fluid into the t-case. Also, the last monkey that had worked on them had not only not torqued the nut properly, he'd installed the wrong gasket between the two boxes. The overhaul & seal kits sold for these boxes will include three gaskets. Use the one that has NO holes in it other than for the input gear and the mounting bolts.
This is a pretty simple repair that can be done with the trans still installed. The boxes both use the same 80w GL4 (low sulphur) manual trans oil, but the natural action of the trans will pump all the oil into the t-case, which will them puke it out from every seal, vent, etc. FWIW, there was a recall back in the day on this. The input gear seal was installed backwards on some transmissions.