If the throttle is not returning to idle position, then there is something that is causing it to stick. I would remove the TB and thoroughly clean it and verify smooth operation before putting it back on the truck. If the throttle shaft has galled in the throttlebody, you may have to explore replacement of the TB. Your best option at that point is your closest junkyard.

The IAC valve is completely independant of the throttle blade. It opens allowing air to pass through an alternate passage in the throttlebody to electronically control idle without moving the throttle shaft/blade.