It only does this when I leave the locking hubs on 'lock'.
Blows gear oil out the vent.
Not so much putting around the farm in 4wd, but running down the road in 2wd.
Sometimes I forget to unlock them, but it's nice to leave them in 'lock' on the road in bad weather.

 

Occasionally there's enough water in the gear oil to make a little pressure when the oil gets warm.

Check oil level, and consider changing it if it's milky at all after being warmed up at highway speed with the hubs in.

 

Thank you, sir.
I'll check it out.

 

I changed the gear oil. It was dirty, but I couldn't tell that it had water in it. I don't think it did.
But that fixed the problem!.
No more automatic rust prevention system.