OK, I just did my engine (major OH) for my 81 CJ and can answer this one from experience.
First you must look for the source of the oil. Normally a rear main is just as you described. Oil on pan, rear of engine, clutch, bell housing, and often all the way back to the drive shafts. Lots of oil. Oil coming out as it sits and moves is what happens if the rear main is hopelessly cracked. Just a single drip, Hmmm probably means starting to go, the more the worse it is. But not necessarily.
My CJ had a huge leak or I should say the previous owner had a huge leak that he was convinced was the rear main and that is part of the reason it was here. The rear main was not the problem, however, it was a bolt problem at the back of the oil pan. Moral of the story: look for cheap dumb things someone has done to the poor thing. This one was pulling the pan and not putting in the right bolts in the right holes. They used the same fasteners at the front and rear corner which are actually 5/16 bolts and the rest are 1/4" it leaked like a storm!
Anyway, putting in a rear main is fairly straight forward. Remove pan, remove the rear bearing cap. Take out the seal in the cap and change it. Hook the other one out and drag it out. May take pliers, screwdrivers and cutting. Clean up the seal groove VERY WELL. Put the new one in. Everyone has their way, probably string and grease is a way, or wire. Pull the one end thru without trashing it! Make sure it is pretty even! Put the bearing cap back and torque, put the pan back and torque it. Probably 200+ at a good shop. Get a guarantee about leakage! Hope this helps. If you have any more Qs about the 258 engine at all I can answer most with very fresh memory!