Pretty rare to have the rear cover leak like that, but there is a gasket there.
Looks more like the pinion seal or one or both of the side axle seals are leaking, and have been for quite some time, and it's dripping down and blowing back to that lowest part.
Unload a can or 2 of degreaser and/or brake clean at that thing and get it cleaned up, top it off, and then drive again and see where the leak comes from.