In My 96 Blazer When the AC is turned on all the air blows out the defrost even though it is set on air. The AC is on and it is cooling it just blows out the derost vent instead of the air vents.
The defrost setting is the default if there is a problem with the blend doors in the dash. Most GM models use vacuum to control the blend doors so an inspection of the AC control and vacuum sytem is in order. You can do a quick check of the system by going to the top of a hill with the AC on and releasing the accelerator completely on the downhill run. This will cause a raise in engine vacuum and usually results in the blend doors working for a short time.
Look around you may find a vacuum hose hanging loose somewhere.