at most i might check it like has been said, but i used to take my trucks off road alot and always had it submerged, never had an axle go that way. my older truck had a breather on a hose, look and see if yours does, you want that as high as possible
if its a toyota i would check it because it has a breather on the top of the axle on left side just to be sure easy preventive maintance and if your towning alot it doesnt hurt to change it check your factory schuedule maintance for your vehicle should tell you when to change it for towing
also, if those breathers get clogged they can cause axle shaft seal leaks. you'll see differential fluid starting to seep out behind the brakes. just an fyi...
