I'm pretty sure this is a rare occurance, but yesterday when I got out of work, I noticed that my front diff vent line had come off. No big deal I think, but when I crawled under the Jeep to put it in, I noticed that the metal fitting the hose attaches to had actually come out of the pumpkin, not just the hose had slipped off. Turns out it's just pressed in, and barely at that, so I used a little Belzona (J B Weld type stuff) to make it permanent. Luckily I didn't loose much fluid, and I had a couple quarts of gear oil on the shelf at home to make the open/inspect/clean/refill deal easy. Take a moment to crawl under your rig and check that this is tight, before it bites you also.

Have fun,
Mark

True, this happens a lot especially to lifted jeeps in the rear. The way Jeep has routed their vent lines for the rear differential wraps around either the track bar or brake lines (dont remember where off hand) but you can just pull it off and wrap it where it is less vulnerable. For the front I see a lot of trucks and jeeps having that line just hangin out, most people dont crawl underneath to check their vehicle, but for those who do notice it instantly.

I know exactly what you are talking about.
In the Aug. 2005 Issue of JP in the "WaterProof IT" section, they talk about it and tell how to fix this problem. They said that is one of the places that water can get into your diff and gears and ruin your oil.