Can anyone provide me with a diagram or precise instructions on how to replace the IAC in my 98 4.0?

IF you have a Chiltons Manual, it is in there! It is located on the top of the Air Intake Mainfold. It is held in with 2 bolts, and has a plug with wires comming off of it. It can be seen from the Drivers side of the truck. Its a little larger then the size of "D" Battery. Before you change this, go to a few parts stores and find the cheapest one, and Order it. (unless you feel that what you pay for is what you get) Because this is an Item that parts stores have to order. I have looked around and have seen prices from $50 upwards to $130. I have also been told that My Engine "Does Not Require" an IAC. Bull Sh!t its doesn't! I think in this case the part stores have the part listed under the wrong thing. I know when you look the part up on-line at AutoZone.com, they say it is not required, but then they have parts that are related that they recommend replacing, and they list a "Throttle Air Bypass". I THINK this maybe what Ford Calls the IAC. Oh well I hope this has helped you. I know i have to replace mine SOON! The Noise is driving me crazy!

Chris is right on. I have had to replace both IAC on my Fords (ranger and Ex). The Ranger would NOT start or IDLE with a busted one. The Ex, after cleaning, sounded like a 400hp shop-vac on wheels. I did not live with that long. I got both from the dealershit, Ranger=$65, Ex=$115...hmm looked exactly alike.
KEEP YOUR RECEIPT..I say this because, like the sway bar links, the replacement was a visibly much improved construction. I have the feeling this may me in a future recall. IT DOES PRESENT A DRIVING HAZARD !!. The Ranger stalled at idle 3x in 5 miles, nearly causing an accident.