Sorry for just getting to this now--
It is not likely the diff breather is clogged on this, the top of it is behind the fuel fill. A major mess of tubing is back there. I would suggest disconnect on the top of the diff to determine if it is plugged.
More than likely something else is wrong, the best bet is to swap out the diff. Josh has plenty exchange units available at a great price so give him a call...
Bear in mind, those diffs don't hold much oil, so don't let it run dry, it won't take much..