i had the adaptor-block o-ring go bad on mine. I was 30 mines from home and blew oil all over the back of my truck.
Dont bother going to GM for the part either, they want like $5 just for the one lousy O-ring. You can go to NAPA and get the same part PLUS all the other O-rings for the entire system (i think 2 are missing) and gaskets you'll need for like $2-$3.
As far as changing it, it was easy to do when I pulled the motor out on the '92, but when I did it on the '93 years ago, that wasnt an option.
I think I even left the propshaft in, but I do remember going all the way in from the front along the oil pan with a long extention and a swivel socket, that was the hardest part.
Also make sure you get all of the old gasket removed. Its kinda shaped like a "D" and half of it usually gets stuck to the block.
also, the 2 bottom bolts will get torqued to 15 ft lbs. when reinstalling. DO NOT overtighten.