I have the same problem, it is not the compressor or the condenser. The air in mine will get warm when driving, but if you put the truck in neutral and rev the engine you can feel and here a door in the system close and now the air is cold again. i am still tracking down the problem. The first thing to check is the vacuum pump it should pump upto 15 and hold without leaking down. if it does not leak down check the vacuum on the 3 dashpots under the dash passengers side. 1 controls max air, 1 controls defrost and the settings on that side of the switch and the last which you have to remove the dash (which is easy, just pull it pops out and disconnect the wires) the others are accessable by dropping the glovebox and a kick panel under the glovebox held in place by 2 plastic rivets. If these all work well check the selector switch theyb are known to leak vacuum.