1996 Dodge Ram vacuum leak

I assume you mean the air flow is changing on its own, not that the controls are moving by themselves. There's a fix for this.

Look for the very small diameter vacuum hose on the firewall right under the back of the hood. There is a small black check valve about the size of two nickels stacked together. This can be replaced with a check valve that has a storage canister built in. It is about 2" long and 1 1/2" in diameter.

This system is actually doing what it is supposed to do. When you accelerate, you lose manifold vacuum momentarily. It's that vacuum that runs the vacuum motors in the dash. When vacuum is lost, as could happen with a broken vacuum hose, the system is designed to default to defrost mode by spring-loading the vent doors. Chrysler's attitude was you might freeze your feet, which is inconvenient, but your windshield will be clear, which could avoid a really bad day.

The dealership I used to work at installed the updated check valve for free for their customers. They didn't even write up a repair order or move the car. They just ran out and popped in the valve in less than a minute. You should be able to find these valves in any junkyard too.

Caradiodoc