It has to be the pinion seal or the cover gasket. There are no seals in the axles at the housing -- the axle seals are at the outer ends, at the hubs. The outer ends on both sides in your photo are dry.
It's certainly not the pinion seal, that entire area is completely dry and grease free.
The axle tubes are pressed into the housing and welded all the way around right? So it's not possible to be leaking there correct?