About a month ago we has some pretty severe flooding here in southern Ohio. Both ends of my road was flooded and I had to choose the lesser of 2 evils if I wanted in or out. I'd say I was easily in 3' of water, based on where I found traces of debre in my radiator. I just took it slow and easy, 1st or 2nd gear at an idle, can't remember which cog. Moved the mirrors to where I could see the rear tires. The hardest part was remembering where the ditches were on a couple of twists in the road ... ... ... ... 3' could have easily become 13' :--)
No water in the cab, and since the truck didn't sit, I'm not too awfully worried about diffs, although they are due to be changed when the weather breaks. From years of 4wheelin, I can say that the worst thing on gear boxes is when the truck is stuck or otherwise broke down and soaks in water. If the pinions are turning, they tend to create and oil barrier around the seal.