Check the seals at the drive shaft and the side seals where the two half shafts come out. I would first make sure that the oil in the differential is at the proper level. How big is the leak? If it is just damp, not that much has probably come out. If it is a large leak, I would be more concerned about the fluid level.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record