Your wanting 700-750 rpm, in DRIVE, both engine and transmission at normal operating temperature--after about 20 minutes of operation.
VACUUM leaks = lean mixture and high idle.
People like to mess with things, especially when they are told not to. Remove the intake tube from the throttle body--three torx bolts. Use a 3/32 allen wrench and back off the "base" idle screw so that the throttle plate is fully closed, in other words back the adjustment screw out so that it is not effecting the throttle plate at all, then screw it in until you see the throttle plate just start to move--it is really only there to keep the throttle plate from slamming fully closed.
You said you cleaned the IAC, but as birchlakeXJ said, have you tried unplugging the IAC with the engine running to see if there is a reaction?
Get a propane torch. Start the engine and using the torch UNLIT follow all the vacuum lines and listen for a change in RPMs. Is your vacuum reservoir intact and the lines running to it? The vacuum line tends to break around/under the battery. Do you have the 231 or 242 transfer case? If you have the 231 then you have a D30 CAD axle with a vacuum spaghetti factory that tends to cause problems, including the axle's vacuum motor itself.
Don't forget to check the EGR system on the DS of the engine and the DS fenderwell.
I would recommend replacing both halves of the vacuum harness just on general principles, especially if it is gray colored--factory original. Napa sells both harness for $21.99 ea, they are cheaper at rockauto.com but not when you figure in the shipping, at least not to Oregon.
PS--89 and 90 are blessed with the vacuum transducer valve, it is located within a couple inches of the EGR valve itself, and they like to burn up against the exhause manifold.
