I would think that the only fluids likely to be above the differential would be either brake fluid (from a leaking hose,fitting or tubing) or diesel fuel from a leaking fuel tank or fuel line. The differential oil has no external hoses/tubing.

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Jeff Naumann
Torrance, California
1972 220D 138k miles (sold)
1982 300SD 263k miles
1989 BMW 750iL 183k miles
1993 Dodge D250 Cummins 5.9 202k Miles