Last night in my son's 02 F250 we had the A/C on enjoying the cool air coming from the vents. When he flipped it into 4WD the air quit coming from the vents and started blowing through the defrost. What the heck? Is this a vacuum problem?

 

Most likely a vacuum problem. Possibly the line at one of the hubs split when activating the 4x4.

 

The 4 wheel vacuum system is tied into the a/c. You will find your auto hubs don't work either, you will have to manually lock them.