My first thought is the engine's rear main seal. Either that or the input shaft seal on the trans.
Possible the oil pan, I guess...don't see what else it could be besides those three.
I have a friend's 79 CJ-7 in my garage right now that I'm converting to an auto trans, and it has a rear main leak.
It had the 3-speed before I changed it.
As far as repair options, if it's the trans input seal, the trans has to come out to replace the seal.
Oil pan, I think on most engines you can do that with the engine in the vehicle.
Rear main...you didn't say what engine it has. That one could vary, and maybe someone else knows about all the engines.