Under $120 to replace 4 o2 sensors from Rock Auto. Likely over $600 to replace pre-cats and cat, certainly over $250 for the rear cat only. Seems I'd start cheap and move forward from there.

That code means the post cat sensor isn't seeing enough difference between the pre cat sensor. Sure, it might be a bad cat but at the same time if the sensor that is 6x over it's expected lifespan is at fault, why spend the extra cheddar you don't need to? If you put cats in, would you leave the existing o2 sensors? Probably not.

Incidentally, the 99 Dakota drivetrain service manual starts with clearing codes, checking for exhaust leaks and age of o2 sensors.