Differential blowing fluid

The front differential on my '88 K1500 started blowing oil out of the vent hose last week. I last topped off the dope about six months ago and it is about one inch below the fill plug now. No driving habits have changed and there have been no sound, vibration, or visible changes other than the fluid dripping out of the vent in the engine bay. The vent is still in the stock location about 2 1/2 feet above the differential level. Anybody ever encounter this before?

About the only way that it can blow out is that it is over full by water intrusion or somebody filled it full.