You can get a differential gasket from one of the internet parts houses that supply parts for these old trucks. I got mine either from Chevy Duty (American Classics now, I guess) or Jim Carter Classic Trucks, can't remember which now. I finally got around to replacing it a couple of moths ago. I just used a little bit of assembly grease and made sure that I had all of the garbage cleaned of the two surfaces of the differential housing before I put it in. holding tight so far.
