It seems Jeeps never really fully make their way out of one's system!
At 16 years old it's a good chance the cat needs to be replaced, but personally I'd start with O2 sensors, engine coolant temperature sensor, and cleaning the throttle body. It's all one big math problem, so if sensors are beginning to go bad they'll feed incorrect input (but not bad enough to show as out of range) which is caught at the bottom line (usually P0420). Usually not preferred to just throw parts at a problem, but they're cheap relative to a converter and could probably stand to be replaced regardless.
Being an '06, read up on the 4.0 OPDA as it's a known issue in its own right, but can also lead to issues with timing and related emissions efficiency.
Are there any drivability concerns? If you live in DC I'm guessing there's minimal ability to do the work yourself, I don't know any shops out that way but wherever you take it make sure they have the ability to live frame the readings to properly diagnose it as being the cat compared to a sensor.