Heat/AC will not switch from defroster

[stoke2te]

Recently bought my truck, and noticed that the heat/ac will not switch from the defroster. Any ideas?

[JSchuricht]

You have a vacuum leak. Likely at the hubs if you have 4x4.

[stoke2te]

I have manual locking hubs..

[Zaairman]

You still have a vacuum leak or a vacuum pump that doesn't work. With the motor off and the key in ON, see if you can hear the vacuum pump run while you change the air selector.

[dieselcrawler]

Had the same problem... hose came off the vac pump. It is located on the pass inner fender, behind the battery. Turns out the funnel I keep stuffed in there knocked the hose off the pump, and the hose was quite lose. Trimmed half an inch off the hose, fresh hose on to the pump, moved the funnel, no more problems.