2002 Chevy Astro REAAR AC COOL FRONT AC HOT

Air Conditioning problem
2002 Chevy Astro 6 cyl Two Wheel Drive Automatic

I have a 2002 chevy astro everything was working fine and then one afternoon my ac was not blowing cold in the front but in the rear it was. i bought a gauge and it said my freon level was good what could cause this/