My AC will go from max AC to defrost by accelerating?

I'm sorry cfiler, my expertise mainly lies in Ford and Chevy drivetrains, but I will answer your question the best I know how.

The heater control in your truck relies on vacuum to move the heater doors in the dash. My guess would be that the heater control isn't working the way it should and allowing unmetered vacuum to bypass it and adversely effect the heater doors.

Someone else here may be able to shed more light on this. I hope this helps.

Indy is correct. Also there's a vacuum reservoir and a vacuum check valve in the system that may be at fault.