I have a 1994 Jeep Cherokee Sport w/4.0 L and 5 speed manual trans

Where the transmission and motor bolt together I have a fluid leak, looks like motor oil but rear main seal isn't leaking. I noticed that at the bottom-center of transmission there is a threaded whole that I don't recall ever noticing before. Folks, this is a 1 owner Jeep. That's correct, I bought it brand new on April 26, 1994. I've done just abut all of the maintenance work on it except pulling the motor. However, I read that the empty bolt hole on trans could be either a 5/16 x 24, 8mm x 1.0, 8mm x 1.25, or 5/16 x 18. So I bought each and can't get none to fit. They will only grab about 2 threads and won't go any further. People, I am at a total loss. Does anyone have ANY idea what size that thing uses? PLEASE, my sanity depends on it, as well as my way to work and home,