The IAC--idle air controller--is a stepper motor that responds to extend and retract commands from the PCM.
Your XJ needs to idle between 700 and 750 RPMs after 20 minutes of driving--manual trans in neutral and auto trans in DRIVE with parking brake set and someone's foot on the service brake pedal.
I would remove the IAC, then clean the throttle body with an approved cleaner, paying particular attention to the IAC port. Cotton-tipped swaps, old toothbrushes come in handy for this job. Then clean the pintle on the IAC--be gentle, they will break and then you will have to get another.
Other things that can effect the idle speed is running too rich--which lowers the idle speed; or running too lean--which raises the idle speed (so that isn't your problem).
Things make make your XJ run rich: engine runs too cold OR the PCM thinks it is running too cold--thermostat or CTS (not the gauge/light sender in the head). Bad o2 sensor. Exhaust leak upstream of the o2 sensor. Bad MAP. Bad MAT.