Ok, the IAC valve stands for Idle Air Control, it can compensate for small vac leaks at idle, and like its name states, it controls the idle speed.
The IAC is located on the underside of the TB, it has a four wire harness attatched to it and it screws into the IAC passage.
If the IAC cannot compensate enough for a vac leak, it may throw a code.
Also, if the truck stalls like you say after the Idle drops, you may have a large vac. leak.