When I accelerate or the engine is under load, the AC switches from vent to defrost.

Any ideas on what would cause this?

 

You have a vacuum leak in the system somewhere. Check between the battery and the firewall. There is a vacuum line that goes into the firewall. This is the vacuum supply line to the hvac system. Check with a small hand held vacuum pump and make sure that the line that goes into the truck holds a vacuum. Then you will know which side of the system to check for leaks.

 

Have you had any success diagnosing your problem? I was having the same problem for the past couple weeks and this past weekend it started getting worse. It now blows the air out near the windshield all of the time and never cycles back to the dash vents. I will start checking it out tonight but it seems to me that if I have such a severe vacuum leak that the dash vents won't work at all then shouldn't there be other symptoms related to the vacuum leak?