If the axle is leaking out of that valve that means the axle is overfilled, or the axle got really really hot. Mine will leak after a track day, then it's fine. I once over filled it by half a quart, and it overflowed out of there too.
yea my guess it is def. overfilled. i get cars in my shop all the time where some joe put way too much gear oil in the rear end and it leaks out there or the axle seals. just take this opertunity to drain and refill with some high quality gear oil like royal purple or something and fill it on a level surface and only to the very bottom of the fill hole. leak should go away.
Yeah, mine leaked outta the axle seals too. Made me mad because it really made my wheels filthy. A bunch of dirt stuck to the gear oil and it was tough to clean.