Yup common failure. My ZJ was leaking from there as well. Its probably the transmission output seal. Might as well do that and the transfer case input seal since you need to remove the transfer case to get to the seal.
Drain your transfer case fluid. Then you are going to need to unbolt the front driveshaft at the transfer case I think its 4 8mm bolts and the rear driveshaft 4 8mm bolts at the rear axle. Might need to use a big screwdriver or pry bar to break the driveshafts loose at the unbolted joints they shouldnt be stuck too bad just a little persuasion. Put a jack with a piece of wood under your transmission and jack it up to where its up to the pan level to hold it in place when you remove the crossmember. Then unbolt the transmission mount to crossmember bolts its 4 bolts in the center of the cross beam under your transfercase/transmission. Then unbolt the crossmember from the frame rails 2 bolts on each side.
Unplug your two wiring connectors on the transfer case and disconnect the vent hose on the transfer case. You can now slightly lower the transmission down by lowering the jack a few inches, this helps greatly to access the top two nuts especially the top passenger side is the tightest to get at. I highly recommend a small or short length stubby 9/16" box end wrench for the top two nuts that connect the transfer case to the transmission. I found the stubby 9/16" 12 point box combo wrench at Lowes for like $8 or whatever, well worth the cost. A regular length 9/16" box end wrench works fine for the other 4 nuts (there are 6 total). You can then return the transmission to its original position or there about by jacking the jack back up a couple inches this helps keep less transmission fluid leaking from the rear of the tranny when you pull the transfer case off the back end. Be prepared for some to come out anyway so put a pan underneath to catch the fluid. Then pull/slide the transfer case back away from the transmission and it will slide off the transmission output shaft. Careful depending on what transfer case you have it could be decently heavy. Id say my 249 case was a solid 80lbs or more and my 231 was noticeably lighter just be prepared for a little weight.
You should now be able to see what looks like in the above picture and remove and replace that seal and the one on the front of the transfer case too while you are at it.