if only blows hot from defrost location only, then there is a vaccum leak from intake manifold to the selector switch. or the vaccum selector switch is defective. if it is a electric selector switch then the mode door is broken, common issue. when the truck if first started, is there a repetive clicking noise? if so then it would be the mode door being broken, warrenting having to remove the dash and recovering of the air conditioning refridgerent, or freon, r-12, or r-134a as this year is R-134a, draining coolant as heatercore needs removed, and removing the entire heating and airconditioning unit, located behind dash on passanger side, disasembling and replacing door. if clicking noise is pressent and time and money, and certified air conditioning mechanic not available, then finding the mode door actuator, or electric motor that turns the door, and unplugging it will cause the noise to stop, but forever have no airconditioning, as default location is defrost. if able to select from defrost to panel or floor location but heat only, then blend door is broke. i believe mode and blend door are same part number, but might not.
also check for recalls and tsb's, or technical service bullitions, as might help fix it. recalls are free, and tsb's are only to help you diagnose the problem, and you would have to pay to have fixed or fix yourself, but having to evacuate and recharge air conditioning system and haveing to have the special tools to remove ac lines it may be a job best suited for a professional. GM UPFITTER Technical Bulletins GM RECALL CENTER