AC through defrost (different problem) not vacuum
On my 2002 screw 4x4 I was having the same problem of AC through the defrost. After spending 3 hours tracing and checking vacuum lines from the firewall all the way through the dash all the way to the climate control. All vacuum lines checked out OK. I took out my climate control (push button electronic type) and remove the vacuum lines from the back. Good main vacuum line. All other switches worked fine, vent / floor / floor-pnl/ defrost ect. I determined it to be a bad switch. I went to the local wrecking yard, got lucky and found an identical unit. Took it to my truck plugged it in and good old cold air started blowing out of vents. This all took me about 3-4 hours and $40.00 to my wrecking yard and I was back in business. In my truck, removing the climate control would take only about 10 - 15 minutes. An easy way to check to see if you are getting vacuum to the unit.