You mean without an extened diff breather or without a breather at all?
It's a good idea to go extended.
http://www.nwtoys.com/tech/differential-breather
It's a fun, easy project.
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It is good practice to stop and let your vehicle cool before doing a water crossing. This will be even more important if you are not able to do the extension to the diff breather and it will be under water.
Wouldn't you want it to be hot when you went in? Heat = expansion = no water in diff?
Came through the mojave ford a couple days ago, water was probably around 18-20" at the deepest point (still below my door sills). Don't know what the standard breather arrangement is on your rig, but I'm guessing it would probably be fine without an extension.
EDIT: Okay just read the extension writeup, didn't realize it was right on the diff housing. That thing will definitely be underwater. I'd extend it if I were you :snorkel:
There is a "one way" ball valve inside the diff breather. When I took mine off to move it up higher I could blow out through it, but not suck air in. Problems occur when there is a problem with the breather valve, (happens often) and water is taken in during a water crossing.