I wouldn't take a chance on differential leaks or flex discs. Flex discs in particular can cause the airbag to deploy when they fail catastrophically. They should be inspected and replaced when wear and/or cracks develop; I'm guessing they checked them out. Depending on your skill level, this is all routine DIY stuff, except for maybe the differential seals. Depending on when your plugs and differential oil were changed (if ever), they are probably also due.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280