Yeah check the IAC. The IAC controls... well The idle.
At first start up, during deceleration and while under heavy breaking. It will allow more air to bypass in the the engine to help warm up the vehicle, to maintain the idle while decelerating because of the loss in vaccum and also loss of vaccum while under braking because your brake booster steals so much vaccum. Thats why it rev's ups like crazy and wants to die all the time. The IAC is not opening and/or closing properly. Do you have a manual for that truck?