Does the temp gauge on the truck hit operating temp? and if it does one thing u can try doing it replace the pump, then get a vaccum sorce, you can use another car if need be with a piece of SMALL hose and hook it directly to the heater control valve this is only to rule that out, i beleave one hose should feel some heat AFTER the heater control valve, if thats all working properly lean towards a blend door.
i had a simular issue at my work a few months ago even tho i work for GM the bleed door on the drivers side was stuck cold(dual zone HVAC system) but the rest of the truck was hot, so i removed the acuator and couldnt move the door with my fingers, then i was informed by the CN employee she use to store her pens in the vent grate, we all see where this is going, i had to bring the 2006 3/4 ton down to her bare bones to get all 6 pens out, and everything from there on worked perfctly.
But the reason i brought that up is i made sure everything else was working properly before i went to this point of removing the EVER from the dash.
And dont forget theres a quick way to access that blend door, which is something ive never done looked at or tryed
