I would start by measuring the amount of vacuum that you have at the actuator itself. Compare that value with what you get at the motor. There should be a small difference, but not much. If it drops by half or more to the actuator, you have a vacuum supply or HVAC control head problem.
If the vacuum checks out on both of the lines in the appropriate modes, then the problem could be either the pin, the door itself, or the diaphram in the actuator. To test the actuator, you can pull a vacuum on one side and it should hold steady. If it drops or you are unable to pull a vacuum, you have a faulty actuator. If, when you pull a vacuum, it takes more vacuum to move the actuator than what is supplied, the door may be hung up or other wise obstructed.
*EDIT* For sake of repetition (well, really cause I couldn't find them in a cursory search), here are the vacuum diagrams and charts for both Auto & Manual climate controls found on the '98+ trucks: