OK - "Year/engine?" I've had to ask this so many times that I should probably just bind it as a shortcut...
If you have the ChryCo HO (OBD-I from 1991-1995, OBD-II from 1996-2001,) the sensor might work, but you'll get a "HEGO Sensor Heater Failure" code that won't go away. If you have the RENIX (1987-1990, pre-OBD,) the sensor will not work - the sensor you've got is likely a voltage generator, and the RENIX system uses a variable resistance with a reference voltage pushed through it. It really does help to tell us what year/engine you've got...
However, I also concur that this doesn't sound like a HEGO sensor failure - I'd start by check anything to do with vacuum (manifold bolts, vacuum lines, MAP sensor, and gaskets, for starters.) Does it level out some once you go above idle?
Loose manifold bolts are fairly common on inline sixes (not just Jeep - all sixes) because the sealing surface is so long. I've found it helpful to returque the bolts about once a year as a preventive measure - I'll usually find one or two loose, and the idle will settle down.
If you want a quick way to check for vacuum leaks, get a can of carburettor cleaner, let the engine idle (but do NOT let it warm up - a lot of gasket-related leaks will go away if you let the engine warm up!) and spray a fog around any vacuum seal and all vacuum lines. If you see the fog get drawn in anywhere, and the engine RPM goes up, you've lessened the search for your leak considerably.
I'd also check the connections for the MAP sensor, the Throttle Position Sensor, and the seal around the Idle Air Control motor - those can also cause the symptoms you describe.
And, when having us help you troubleshoot, it's really helpful to give us all the information you can - starting with vehicle specifics. Putting a line like:
88XJ/4.0/BA-10/D30/D35 (3.07)
Is a huge help - read it as "88 Cherokee w/4.0, Peugeot BA-10/5 transmission, Dana 30 front, Dana 35 rear, 3.07 gears" and you're on the right track. If any of us here are psychic, they're hiding it well (me, I'm psychotic, not psychic...) Also detail any engine mods - if you've done a stroker, how long ago and what you've done (you can list that as "4.0 + 258 + .030" - 4.0 block, 258 crank, bored +.030" - or similar.) There is no such thing as "useless information" - we either haven't figured out what to do with it yet, or it comes in handy later. Making everyone go to your profile is time-consuming - and for people like me with several vehicles, no help anyhow. Help us help you.
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