There is no idle screw on the TB of an XJ. There is, however, a "keep the throttle plate from slamming shut and damaging the throttle body screw", and that screw is sometimes called a "base idle screw" but that isn't what it does.
The IAC is a stepper motor and it will handle the idle fine. You should, however, check the "base idle screw" and make sure it is set properly. Remove the intake tube. Use a 3/32 allen wrench to back out the "base idle screw" until the throttle plate is fully closed, then turn the allen screw back in until you can just perceive the throttle plate moving AND THEN STOP. Put the intake tube back on, and let the IAC do its job.
If the IAC isn't doing its job, then pull it and clean it with throttle body safe cleaner--it will state that on the can. Clean the throttle body cavity too. They both get pretty nasty. While you are doing all this you should test and adjust the TPS--if it is telling the ECU the throttle is off-idle then the IAC won't do its job.