I'm a bit baffled. Every M/B I've ever owned used tapered threads on the drain and fill plugs for the differential. There never was a washer to forget. Am I missing something here?
Older model Benzes used diff plugs that required an aluminum seal. W123, 126, etc.
I have seen many very worn out drain plug seals that were leaking because the jiffy jockeys apparently don't replace them. Some actually are squeezed out around the head of the plug.