05 F350 4x4 6.0 vacuum leak
 

Pretty sure I have a vacuum leak. Everything works great until I try to switch to 4x4. My air starts blowing out of the defrost only. Any suggestions where to start or common easy locations that seem to have this problem a lot? Just bought it and the first time i tried 4 low it worked fine. Then a few weeks later I tried 4 high and the air went to the dash.

Just saw the sticky thread for the Hubs and vacuum leak. Guess I should've looked there first.