I know this may not be the solution your looking for..... But it could save you money,.

You could also as a cost save just lower the oil level in the diff and not worry about it. The diffs have a tolerance on how much oil they need and just keep the level below the diff seal and monitor occasionally for more leakage. I know this isn't fixing the problem, but it is a lot cheaper, and their isn't a lot of risk in running the diff dry, since the case seal is not leaking. Just keep an eye on it.

I also have found that different diff fluids more easily leak than others. It might be worth while to try a couple of different diff fluids, BMW's, redline, etc. to see if that stops the leaking.