I'm a newbie to this forum so I'll get to the point with my question.
I was wondering what would cause my air conditioning system to switch from the upper dash vents to the defrost vents when I am running the air conditioner on auto, when I switch from 2wd to 4wh or 4wL? When I switch back to 2wd it takes a little while then the air condo starts blowing out the dash vents. It's a 2007 F350 Harley Davidson when I bought the truck last year,it needed some front end components replaced and had the"factory" plastic locking hubs and my mechanic suggested that I replace these with new warn hubs so I did. I'm thinking possibly this truck might have had the hubs replaced before I bought it and was wondering if there were any models that had vacuum automatically locking hubs that maybe I should be looking for a vacuum line somewhere before I tear into the dash looking to kill a gremlin that ain't there.

 

From what a lot of others are talking about, sounds like a vacuum leak.

 

More specifically the vacuum leak is the ESOF hubs or the system lines feeding the hubs. The hubs have an o-ring seal that fails. The lines typically break at the axle by the barbed fitting but can crack anywhere. The default with loss of vacuum is the defrosters so you can at least see if the windows fog and not seek legal counsel.