Why would the AC in my 1997 ford expedition change to defrost during acceleration and then back to AC once I let off the gas

Try to perform EATC on-board diagnostics:

https://www.askmehelpdesk.com/cars-t...ml#post1999642

The controls are operated on vaccumm, When you accelerate the vaccumm(sp)drops and the blend door starts to go back to the off position. When you let up the vac comes back and puts it where the control position is set. This can be considered normal.

You shouldn't have switching back and forth if all the vacuum lines are intact and the vacuum tank is holding proper vacuum.
That tank is designed to provide sufficient vacuum during heavy throttle applications.