The OEM Mopar O2 sensor ARE NTK!!!!!! Only difference is you'll pay less for them in NTK boxes than you will in a Mopar box! Just don't use BOSCH O2, our trucks don't like them or at least not for long. Lots of posts about this all over online. I had them in my wrangler for a while, had to replace them once a year like clock work.

 

Also being a 3.9 how long since timing chain was replaced? Being an odd fire engine they like to eat them around 125-150k miles. Retarded valve timing will have an effect on emissions. Don't cheap out on the replacement. Yeah some have a tensioner on the slack side, but that only prevents the loose chain from slapping the inside of the timing cover as it stretches and wears. Doesn't prevent the retard of valve timing compared to piston and crankshaft position.

 

You may have missed this........and I used to purposely retard timing (slightly) on my old cars back in the day so they WOULD pass.