what is a good transmission fluid sealer a small leak.
Automatic or manual?
It depends on the leak, if the pan is leaking I would remove and reseal the pan.
Replacing whatever gasket, seal, or o-ring that is leaking, will generally stop the leak. 
Good luck with your Cherokee ,find out whats happening and if the amount is small ,it will probably soon be the least of your worries (seen to many of these things go to the wrecking yards for whatever reason )I sincerely hope yours is a good one ,my next door neighbor just got done putting a motor in His wifes work Jeep ,the first replacement motor was total junk.
On the other hand my brother had a 4th hand early 90s plain jane Cherokee(manual ,no AC )that gave exempelary service .
If the rear seal is bad ,thats an easy fix ,you can do it yourself ,if you have an auto and the pan is leaking ,pull the pan drain ,new filter and gasket and you are good to go .