I've used ""The Right Stuff" to seal my rear diff a few times. Worked great until I had to service it again a few years later. That stuff was hell to remove and clean off.
I now use a re-usable rear diff gasket called "Lubelocker" that needs NO sealant or adhesive, and installs completely dry. The thing is AMAZING. I've already re-used it at least once. Just clean it off with brake clean, along with all mating surfaces, and reinstall it. They make them for rear diffs and for transmission pan gaskets. When I get around to servicing my transmission again, I plan on buying one for my transmission.