If your drivetrain springs a leak, it could be that the breather/s are blocked. After plugging the leak, it’s important to clear the breather.
All drivetrain components (front and rear axles, gearbox and transfer box) have breathers. They allow air to escape when oil and air get hot and expand and allow the air to go back in again when they get cold and contract.
If the breather gets blocked, the components will breathe through an alternative route. The path of least resistance is usually a rubber seal or an O-ring.
If you’re operating a 4wd in difficult environments—hot and dusty, for example—it’s a good idea to check the breathers more regularly. We do our fleet every other service (10k intervals). We work our drivetrains quite hard and train in dusty conditions, so when the components breathe out they breathe out a mixture of air and oil vapour, which coats the inside of the breather pipe. When they breathe in, it’s a mixture of air and dust and, quite quickly—due to narrow airways—they get clogged up.
It’s the same principle as knowing the wading depth. If the water is too high, the cooled (by water) components breathe in water. Water, being a poor lubricant, will damage bearings and gears.
One recommendation I’d make if using 4wds in difficult conditions, as above, is to upgrade the breathers. ARB makes a kit for this—generally just for the two axles—but it’s important to do the two boxes as well. The airways are a larger bore, fitted higher, and the pipes go into a filter box. Less chance of blockage and bad stuff being breathed in. Happy days.