Anyone know why my 05 eddie bauer expedition's automatic a/c switches to the dash defrost under any acceleration then back to the vents when I let off the accelerator?

 

Vacuum leak is usually the cause of that type of unintended switching. Common place for an issue with the vacuum lines is under the battery tray.

 

Mine is doing or was doing the same thing. Now the leak has gotten bad enough that it just blows through the defrost all the time. I figured it was a leak but didn't know how to find it. I'm going to check those lines under the battery tray like you said alwaysfords41 in a couple of days when I get a chance. If I can't physically see the leak is there a way to find it? Some trick to finding it would be great. I'm hoping it will be a gaping hole since it is bad enough to keep my air running to the defrost all the time. That sure will make finding it easier.