Diffs and t-cases are vented. The trans is vented as well. Typically, the trans will have a check valve that lets air blow out, but closes to prevent water from getting in. The diff and the t-case may or may not have a check valve, but if they do not then they should vent above the high water line, which should be lower than the throttle body.

You can follow the vent hose to the component that it is venting, if you really want to know what a hose is for.