a/c blowing out defroster vents only

Check the electrical connector on the vacuum pump under the hood, I had the same probelm not long ago and all that happened is that the electrical connector had worked itself loose from the vacuum pump. I dont know if this would be your problem too or not but it is worth a try.

Try and check for vacume under hood passenger side next to Evap coil you'll see a white vacume tube going to a cylindrical canister wich operates a door for heater or evaporator if so look under dash for
another canister like the one outside just a little flatter should be the one on the passenger side you may have take out remove hose carefully with truck running you should hear a sucking noise try in all setting's to make sure you got the right one
remove (vac canister) put your mouth on tube should have resistance and arm should move


Let me know if this Help if not i got a couple other things for you to try