Hello, my name isXXXXX you for allowing me to assist you! High Nox is a common issue on the 4.0L I am a smog tech in Northern California and see this a lot. What I do before replacing the cat is to 1 perform a decarbon procedure (which you have already done), then 2 do an injector flush. This usually gets the Nox down enough to pass. Now if the check engine light is not coming on and setting a "fuel trim too lean" code, then it is probably not that lean. Also, there is no serviceable fuel filter. It is built into the fuel pump module. Now it still may be the cat, because it is basically a 3 way cat and the Nox reduction section may be worn out. The other things that will cause high Nox is high combustion chamber temperatures, so make sure the cooling system is operating correctly. Premium fuel burns cooler, and sometimes that will help. Any exhaust leak before the cat will cause high Nox, so verify that there are no exhaust leaks. Even the tiniest leak will increase Nox. If these things I have mentioned are all ok, then go with a new cat.
I hope this information helps, and let me know if you need further assistance with this.