I have burned up (broken inside shell) 3 Cat converts in the past 5K miles. I have 2000 TJ (55K miles, original owner) with 1 upstream and 1 downstream sensor. No codes thrown, runs/idles fine ... but after a few trail runs the CATCO replacements start to rattle (and break up).
Spent about $1K trying to get to root cause and trusting my rig again. I had it stressed tested on dyno and it leans/trims out fine. Took it to some mechanic's mechanic (w/aftermarket sensor tools) that show nothing is wrong (except that I am missing 2 sensors).
Finally, took it to dealer to confirm old computer software IS working, but they noticed slow response in upstream O2. Still, they said the likely problem is too much air flow from aftermarket cold air intake.
This forum said ... "the dealer statement about CAI allowing too much air is junk".
I am thinking to replace both O2 sensors, but a 2 sensors 2000 TJ is an "odd duck", as stated earlier. Even the NGK site is expecting 4 sensors on this duck.
1.) Can I get a definitive NGK part match??
2000 was the odd-ball year where some models had the earlier system with only 1 cat and 2 O2 sensors, whereas later models had the 2 pre-cats and the 4 sensors. If yours is the earlier style with 2 sensors, follow the O2 sensor guidelines in the original post for the 1999 model is what I would suggest.
And yes, the CAI intakes aren't any good for these 4.0 engines. I know so many people want to believe they are, but that much has been debunked so many times. Spare yourself, your engine (and your cats) by switching back to a stock airbox and air filter. Even those K&N filters people use in the stock airboxes let too many contaminates through.