There are several reasons why the transmission will help rustproof (besides worn seals/yokes and oil leaking through the splines of the shaft and by the pinion nut). One is that there is no seal between the transmission and the transfer case, so if you go up or down a very steep incline you actually transfer oil from one gearbox to another. There is a sealed bearing that can be installed to prevent this, but in the end the oil cant go any higher than the balls of the bearing.
Second is that the bolts between the transmission and transfer case can work loose, then leak oil between the two boxes.
Last, the front seal on the transmission leading to the clutch bellhousing is merely a felt seal rather than a positive rubber lip seal, so it will always seep a little.
But if you think a Jeep is at all bad, go buy any year Chevrolet......
