Just curious as to the location of the IAC on the 4.0 ohv, I have already cleaned the MAF sensor as per the directions on here and smoked my tires in 2wd out of my driveway (kudos to whoever discovered that little trick *grin*) but I'm not sure if the IAC is the black plastic box on the side of the throttle body or the aluminum tube type of thing that is just behind it on the side of the intake. I don't have a whole lot of experience with fuel injection systems. I was still playing with carb motors until I got my Exploder and I love my truck now. Any help finding this IAC thingy would be a great help. And if anyone has a set of automatic front hubs for sale for a 91 XL lemme know. Blew em first time in the mud, 4-wheelin rocks!
The "black plastic box on throttle body" you referred to is the throttle position sensor. Rearward of that and mounted to the intake is the Idle Air control valve. It is mounted with 2- 8MM screws (i think, could be 10's though) If you still have the black plastic cover with 4.0L lettering covering the throttle area, it snaps around the IAC.
Hope this helps