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Well, I can't think of any other seals, but there is the possibility of slight seepage via the splines on the companion flange as the oil seal bears against the outside of this, so there is a theoretical leak path. Maybe they used a bit of "goo" when installing the washer before the securing nut was fitted.
on the X350 differential do you need to replace any other seals?
If you're not disturbing the output shaft seals (Part 5), then the only other thing I can think of is the rear cover, but the Electronic Parts Catalog doesn't show a gasket for the cover, so it must be sealed with RTV silicone. My recommendation for that would be Permatex Ultra Grey RTV, since it is stronger and more rigid than the Black RTV. Here is the parts diagram from the EPC:
