Its not leaking, right? I agree it sounds more like normal casting marks. Not a weakness, not an issue.

More than a few people have abused their diffs and broken them, but I am not aware of anyone ever cracking the cover, without also grenading the internals into shrapnel.

If I were you I would fix the diff if and when it ever really breaks, not before. But if you are paranoid, flush with cash, and want to make sure you won't ever be stranded due to diff issues, get a puddymod diff upgrade.

 

I also have at least 4 of everything you need to rebuild/fix what you have if there is indeed an issue that needs fixing.