A diff breather valve is usually installed at the highest point of the diff, in other words on top, to serve as a vent for a release of gasses caused from heat. It can get blocked if the vehicle is used on dirt roads. Usually it can simply be screwed out and cleaned. I would advise that you replace it with a new one and remember to take special care that no dirt enter the diff if you should replace the breather.
I also wouldn't be surprised if the leak you mention, persists, as I cannot see why the diff would choose a particular oil seal out of three as a substitute for the breather valve. Breather valves are seldom the reason for a pinion or side shaft oil seal to go South. Breather valves had the nasty habit on older 4x4's to suck water into the diff, because the sudden cooling down of a diff if you do some wading, causes a vacuum, which is filled through the breather with water from the stream you're traversing, otherwise they are quite well behaved and I believe manufacturers have by now solved the drinking habit too.