Ok, sounds like you could have a number of different issues going on.
First, and foremost, clean secure grounds are ESSENTIAL for Renix era XJs to function. Poor grounds will cause more issues than you could ever imagine. There is only ONE unibody ground--a POS braided strap from the head to the firewall--if that is the "wire" you wiggle, well, it would explain a lot. Replace the braided strap with a 4 or 2 gauge cable. STRIP the metal on the head and firewall down to shiny bare metal. Treat the metal with an anti-corrosion agent from any paint/hardware store. Make the connections TIGHT. Ok, now lets move to the OTHER critical Renix grounds. Attached to the dipstick is the ground for the ECU--critical--the ground for the ICM/coil--critical--the ground for the o2 sensor--critical--and some others. Same as the cable for the unibody ground--strip to bare shiny metal, treat with anti-corrosion agent, make the connection TIGHT.
Now, the IAC mounted in the throttle body is the "muscle" for the ECU when it comes to idle speed. The IAC is a stepper motor that responds to ECU commands to extend/retract and change the idle speed. This can all be screwed up by poor grounds--starting to see a theme here? Ok, as a high idle can be caused by an intake leak--leaner mixture, higher the idle--too rich of a mixture can cause a low idle. Your XJ should idle at 700-750 RPMs after 20 minutes of driving (fully warmed up), with the transmission in DRIVE with the parking brake applied and someone's foot on the service brake pedal. Ok, things that make the engine run rich--poor grounds screwing with the sensors/ECU (theme continues), an exhaust leak upstream of the o2 sensor or a poor o2 sensor ground (theme), vacuum line fell of the MAP sensor on the firewall or out of the throttle body, a bad CTS (coolant temp sensor in the driver's side of the block under the manifolds), bad FPR (fuel pressure regulator), bad thermostat (engine running too cold).
Here is everything you need to know about Renix sensors and how to test them with a multimeter:
http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm
Here is an excellent article on upgrading the charging system:
http://www.olypen.com/craigh/charge.htm
And, not to forget the EGR system including the vacuum transducer valve:
TEST your EGR system, info from 90 FSM:
Warm the engine to normal operating temperature.
Operate engine at idle speed.
Check vacuum at solenoid vacuum source.
Disconnect the hose and attach a vacuum gauge to it.
Vacuum should be at 17 inches.
If vacuum is low, check the line for kinks, twists, or a loose connection at vacuum connector on intake manifold.
If vacuum is ok, remove gauge , reconnect the line and proceed to next step.
Check vacuum at solenoid output port.
Disconnect the line and attach a vacuum gauge to output port. Vacuum reading should be 0 at this side of solenoid.
If vacuum reading is 0, leave the gauge connected and proceed to the next step. However, if vacuum is present check solenoid/ECU operation with the DRB-ii tester.
Disconnect electrical connector at solenoid and note vacuum at output port. Vacuum should now be present at output port.
If vacuum is present, proceed to EGR valve test. However, if vacuum is not present, replace the solenoid. Test the EGR valve as follows:
Leave solenoid electrical connector disconnected. Note engine idle.
The engine should idle roughly or stall. If this occurs the valve is ok. If the idle does not change, proceed to next step.
Disconnect hose from the EGR valve and connect a hand vacuum pump to EGR nipple.
Apply a minimum of 12" of vacuum to the valve and note engine idle. If engine now idles rough inspect the vacuum line between the EGR valve and the solenoid.
If the idle did not change, remove the EGR valve and inspect the valve and the exhaust passage in the manifold for blockage, repair as necessary. If no blockage is present replace the EGR valve.
TRANSDUCER VALVE:
Disconnect the transducer vacuum lines and the back-pressure line (bottom). Remove the transducer. Plug the transducer output port. Apply 1-2 pounds air pressure to the back-pressure port. Apply a minimum of 12 inches of vacuum to the input port. Replace the transducer if it will not hold vacuum.
Well, that should keep you busy for 5-10 minutes checking and testing all of that.
Also, are you still running the stock OEM injectors--the Jeep fire-starters? Read this JP Magazine article:
http://www.jpmagazine.com/techarticles/engine/154_0811_preventing_jeep_engine_fires/index.html
Also again, don't forget to check and adjust the TPS to specs.
Also again and again, adjust the throttle pressure cable:
http://www.naxja.org/forum/showthread.php?t=1015963
Keep the power/comfort switch in the power position.