It is switching to the defrost vents when you are accelerating the car, then switches back to your front vents when you let off the gas, right?
If so, it is caused by a vacuum leak that is most likely on the vacuum line that runs from your intake manifold, along your firewall, and down the left fender toward your battery, terminating in a container below your battery called a vacuum reservoir.
Most of the time, your leak will be because the vacuum line that goes to your HVAC becomes brittle and often cracks (it looks like the image below, although this is from a Cherokee rather than a Grand Cherokee). It is a 1mm ID x 3mm OD hose, if I recall, about the size of a fuel line for garden equipment. If that is your break, you can replace it or (alternatively), put some shrink tube over it and create a new exterior sleeve.
If that's not it, you would need to search for the leak visually, use smoke to find the leak, or use a mityvac to test for the leak.