Ryan, only the bell housing and the transmission are pressurized after you plug the bell housing and pull the fording handle on the dash. The breather valve by the shift lever will vent the air continously when bellhousing is pressurized (to about 2 psi, like Ken said). The boot around the shift lever must be tight in order to hold the pressure.
Outside pressure will close the breather valves on the axle housings and transfer case. The batteries also have vents that are waterproof to about 4 ft.
In water over 2 ft, the crankcase breather must be extended up and above the hood. Compressor intake is connected to the engine air intake canister.
After fording: You must take apart all wheels and check seals, there is no other way to confirm that the seals held or not. Big job. Do it within days. This goes for fording any water that will submerge the axles.