It's also a good idea to check the diff vent (pt# 42110C in Tim's picture above), before jumping into big repairs.
If the vent is plugged, it will build pressure inside the housing and "leak" wherever it can.

Both pinion and pumpkin seals are a pita to change, so it is worth spending the time to check out the vent.

The metal cap over the fitting should be sloppy loose, if in doubt, service it.

I recently had a leak from the fill plug, that developed out of the blue.
Seemed weird, especially when a new gasket "failed" again in less than 2 weeks.

When I checked the vent, I couldn't move the cap at all.
Unscrewed the fitting (14mm deep socket), soaked in seafoam for a few minutes, freed it up and blew all the crude out and reinstalled.

10 mins work and no further problems.

BB