On an 73-87 A/C truck, only the heater blend door is cable controlled and the rest of the doors are moved by vacuum motors. Once you remove the glove box and get access to the cable end at the heater plenum, disconnect it and see if you can move it back and forth with the dash lever. If it won't move, lubricate it as CRM suggested, or go online and buy a new cable. If its the door itself that is binding, you need to look inside the plenum.
If your truck is typical, cleaning out the heater plenum is like an archaeology dig. I found a rather large mouse nest (they like that insulation from under the dash), a pencil, a pen, a couple bolts, matchbooks from bars, and a few live rounds of ammunition from a deer rifle. All if it dropped very nicely down the defroster vent when someone set it on the dash.
Also take a close look at the small, hard vacuum tubing that runs under the dash, and see if any of it running to the vacuum doors is broken. Each section of hard line has a short rubber section on the end, or the device it connects to has a rubber nub where the tube plugs in. At the heater controls there is a large molded rubber connector that consolidates all the vacuum lines and connects them to the correct ports on the controls. You can repair small line breaks with a short section of rubber vacuum hose, and most auto stores sell replacement GM plastic hard line if you need a longer run.
Its also fairly common that someone has disconnected the vacuum supply line where it connects to the reservoir/canister under the hood, or has disconnected the line from the reservoir to the intake manifold. In some cases they have taken the entire vacuum reservoir out. Many guys get carried away with "cleaning up" under the hood and they take off the HVAC vacuum lines because they think its something related to emissions controls. Stripping out things under the hood without understanding what they are for can get you in trouble.
Bruce