[ToughBowtieTruck]

There is a vacuum line that is routed from the selector switch, through the firewall and to the engine's intake. Could be the vacuum line has a crack in it, or has been accidentally unhooked/blocked off if the emissions system has been modified at all (probably not from what you say). Not every truck has the exact same port for the intake. Most likely, behind the throttle body is where the line hooks up; usually there is a vacuum tree there where a few things are hooked together.