f150 strange transfer case noise

i have 2003 f150 4x4 automatic truck, and its making a strange whirring noise like gears winding up. it sounds like its coming from the transfer case. it does it in both 2 and 4 wheel drive. when you put it in 4 low it shifts really hard now. it was the first time we took it off road and it started doing this. it was in water at a couple points, and the rpms were kept high at alot of points to keep from getting stuck in the mud. any ideas on whats goin on and what can be done to remedy it? thanks in advance.
it only does this when there is a load on the engine, if you dont have the gas down its not as noticable, but when you put studdy pressure on the throttle you hear it wind up. it doesnt make it at all when stopped or in neutral coasting