I take it you have shift on the fly 4x4, with the swith on the dash. Sounds like a vacuum problem, as the 4x4 and a/c controls are both controlled by vacuum. Wish I could be more help.
I had the same problem with my 2000 Excursion. It turned out to be the 4 wheel drive actuator it is located either under the hood or near the front axle. Basically it was leaking and taking the vaccum from the A/C system causing it to change where the air comes out the dash. I know it did not involve removing any of the dash though. As everyone stated it is indeed a vaccum problem, and I had the same exact symptoms as you. Sorry I cant be more specific with where the part is or the name of it, but I had the dealer fix it. It wasnt too costly if I remember. Hope this helps!
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