Maybe the ECM will fix it. The tach going crazy with no other gauges jumping around sounds like the crank or cam sensor, but could possibly be the ECM. I think everyone with a ZJ any year model should have a spare ECM/PCM for troubleshooting purposes. The OEMs are at the end of their service life.
I found that the connection to the cam sensor was flakey on mine. Best thing to do is wiggle all the connections around while it's idling and see if you get a stall.
Quadratec Gets there parts from CROWN Automotive, who are a dealer of nothing but Jeep parts, and they are all factory replacement I am told. But they dont sell to the public, have to be a business to buy from them.
Rubicon Express 3.5",5.2 Swap,LP10K Superwinch/7/16 syn line,RE Adj Track Bar and Drop Bracket,Hercules Trail Digges 33x12.50, D Windows ,HI Lift,Custom built Rad Support,D44 Rear w/ spool,HP30Aussie Locked , Alloy USA Tube seals, Custom rear bumper,More to come.
Well, I got my PCM back and put it in and cleaned all my ground wires and even replaced one of them....nothing! It started up great and ran for about 15-20 min. then tach went crazy then shut down. CE light came on so I checked the codes with the 3 turns method and NO codes. ??? Do these GC's have an ignition module? My van did the same thing and that fixed it years ago. HHHEEEEELLLLLLLPPPPPPP!
O.K., so I got my computer back and it started up and ran GREAT! BUT......I had to disconnect the battery to fix some of the wires on the stereo (just being safe) and when I finished I reconnected the battery and heard a little pop. Now it's back to running like crap and shutting off. I know what your thinking...why didn't you fix the radio when the PCM was out being fixed? Stupid here has been so busy doing other things that I forgot about it. So this weekend A friend and I are going to fix it. Anyone had this happen?
i have '98 4.0 with all the same probs as yours... went through changing all the same parts, including the new computer ($$$). jeep drove fine for about a day with the new pcm (i was so freaking happy), but then it stalled again in an intersection (my heart dropped through to the floor). lucky me i got a code, so i limped straight to AZ and got the O2 lean bank reading.
i put in a new front (mopar) O2 sensor (rumors suggest they don't care for the less expensive ones for very long), and all my problems have been solved for the past 6 months. no more stalling, hesitation, etc.
btw, i didn't bother to change the rear O2 sensor yet (i'm cheap), but i'll get to it sometime this summer.
hoping this might help you, too. good luck.