first things first, run some kind of bg injector cleaner, run rite, seafoam, or what have you though the intake to clean out the system. if your truck has been running rich then you may have coated the bricks in your converter with gunk and they need to be cleaned out so that your catalytic converter can function properly. once you know that the fuel system and converter itself are clean you can clear the code and see if it come back. cats dont just go bad and the 'stuff' inside doesnt get used up, if they did the scrap yard wouldnt give you anything for a 'bad cat'. you need to get into the data stream and see what outputs your sensors are giving and find which ones arent where they need to be, whether its temp sensor, mass air flow sensor, upstream 02 sensors becoming lazy or dormant, etc. something is causing the cat to not be able to do its job and the down stream 02 sensor is picking up on it.