Common pinion seal leak. The seal should be available at your local Napa or specialty aftermarket source. Remove the driveshaft, remove the large nut on the shaft, tap the flange off the shaft. It's probably a good time to drain the oil as most of it will run out anyway. Remove the seal, clean up the components, install the new seal with some sealant on the outer edge and some grease on the sealing surface. Reinstall the flange & nut. Torque the nut down, install the shaft, refill the diff.
Fred
