The AC Idle up valve could be routed wrong, or plugged. Does it make a difference if it is screwed all the way in or out?
Also, does your truck warm up correctly? Even if it reads 40% on the temp gauge, have you felt the upper radiator hose while it's warmed up? If it is really ~190 degrees, then you wouldn't be able to hold onto the hose for more then about 2-3 seconds.
I ask this because the A/C is supposed to not work the idle up when the truck is cold because it is still using the cold idle. It also incorporates 4WD with the system, but I have never seen a difference. I've used AC in Marlin's 4WD truck both with it cold and in 4WD and it worked just fine
