I have a 93' can and chassis with a 7.5 non a/c truck that lately is blowing cold air while driving. Buy warm when it's sitting.
the trick doesn't over heat, no leaks, fluid is full. Heater core is blowing hot air(took cover off inside and the core is hot). But was wondering if if the valve on the firewall box is bad as it only moves by hand and blows warmer air on the dash vents when it is depressed.

 

Sounds like the blend door is malfunctioning. Letting cold air from outside get in the cab.
That should move when you change from COLD to HOT. If not, then something is wrong with actuator or controls.
Can you feel vacuum on the line when you unplug it and move the controls inside?

 

What are your outside temps?

 

Is the vacuum line hooked up and working? Mine was broken.

 

on our 94 F-SuperDuty it was a stuck blend door in the air box due to six dollars and change, a few pens and pencils, and 20-30 cups of coffee falling down the defroster ducts.
too me about 45 minutes to clear the air box out and get the blend door working again.
after that the guys started complaining they had to drive with the windows open because the heater was cooking them.