This happened to me as well. Finally I noticed that a cap in the center of the driveshaft yoke was missing. Had a piece of metal welded over it and it no longer leaked. To see the cap you can try to look through the ujoint into the back of the yoke. If you can see in the yoke it is missing.

89 2dr 3.0 AT, manual hubs. 33x12.5 MT
gen 2 coils,Cranked front tbars
Dons rocker protection, Front/Rear Bumpers. ARB rear locker, 2" BL, Beefed up skid plates/trailing arms. Gen II uca, 2 Bouncy seats www.youtube.com/TagGeorge