I have a 03f250 6.0 4x4. I use the 4x4 maybe 3 or 4 times a year. My truck has the shift on the fly, automatic 4x4. While using it a couple months ago, the a/c was blowing thru the defroster while in 4x4. Yesterday I had to use it and I could not get the electronic 4x4 to engage. Finally I got out in the mud and locked the hubs. By that time the 4x4 light came on and everything worked fine. When I was done, I unlocked the hubs and it took a while for the 4x4 light to go off. Like, longer than the 15seconds or whatever it saids in the book. Bottom line is: 1. I don't know if I am engaging the 4x4 correctly. 2. A/c is blowing out the defroster while in 4x4. ( I think this may be the vacuum pump, but I am unsure). If it is the vacuum pump, how hard is it to change and how much would a dealer charge to change it?