In the wheel hubs. My bad I read this quick, the part you are talking about, looks like a valve, yes? Normally when you see 80/90 or gear oil coming out of the QRF or quick release valve, it is a sign of a ctis seal that is allowing air into the diff housing. Most the time, it blows it out of the breather. There is a check valve on the wheel assy, but that is so that the air doesn't leak from the tire back into the system.
Hope this helps.