I have a 90 cherokee, and when I coast or slow down, the heater air flow shifts from below the dash to the dash vents. When I accelerate it retrns to the floor vents. The same thing hapens with the AC.Any ideas?

Sounds like a small vacuum leak somewhere. When you decel, the vacuum can't keep up with the leak, when you accel the vacuum overcomes the leak...

Look under the front bumper, PS. Inside the bumper is a plastic"ball" that should have a vacuum tube going to it. This holds your vacuum for your vents. Check the lines to and from for cracks or leaks.