There’s several books about hopping up Ford motors at most larger Barnes and Noble bookstores. Go to their “Transportation” Section and take a look. Your local public library probably has something of interest there on this topic too. However, unless you are doing it just for your own interest, I doubt the payback will be worth the cost in $$ and your time. You’d be better off trading in your existing truck and buying another truck with a more powerful engine, if that’s what you think you need. If your state has stringent emissions standards, that’s another reason to avoid modifying your existing truck.
Inconsistent idling can be due to the idle-air control valve like you say. All that thing does basically is the same as if you pushed the gas pedal down a little further. It is supposed to bump up the idle when the engine is cold or if an extra load is put on the engine, like if you turn on the AC. I donn’t think the IAC is usually able to get the idle speed to 2500 rpm though, so unless it’s really broken, I doubt that is the problem. Does it idle-up ok when cold, then gradually drop the idle speed as it warms up? If so, then the IAC is probably ok.
More likely some kind of air leak. It could be coming in from anywhere, like at a worn throttle valve housing. But like you say, it is probably coming in through a vaccuum line. That’s the first place to look. Vaccuum lines originate at the intake manifold, some before the throttle valve, some are after. Since the problem is idling – not hesitation on acceleration – it is more likley one of the “after” vacuum lines that is the problem. One of the gadgets one of these vacuum lines is controlling has probably sprung a leak. It could be anything from the brake booster to the EGR. Mechanics have a hand-held vacuum gadget, which they use to test each device to see if it holds vacuum.