There is only 4 O2's 2 pre-cat and 2 post-cat. The rear O2's are not necessary to run and are only used to monitor that your catalytic convertor is working as it should. All four work in conjunction to communicate to your computer (PCM) in order to give you the best fuel economy and performance while being EPA friendly and quite frankly decreases performance and fuel economy to be more environmentally friendly. The catalytic convertor only hurts performance as it is a restriction of airflow but it is a EPA requirement as it sole purpose is to burn un-burnt fuel leaving the engine to keep all the happy tree huggers quiet. The front O2's are the only ones that are required as they take the exhaust straight from the engine before being re-burnt in the cats and tell your computer to adjust fuel trims and AFR in conjunction with your Map sensor and MAF sensor in order to run most efficient. Like I said previous post, old stock cats can cause running problems as the packing deteriorates or bust up or plug up inside the cat causing the rear O2's to send codes. Most of us with performance mods remove them completely and tune them out. If your state doesn't require a state inspection your best bet is to remove them or to replace them with fake straight through ones or just a exhaust pipe section.
The tune in itself will be a night and day difference in performance and you will see better gas economy as long as you stay off the skinny pedal... You can get it tuned at any dyno tune shop for $400-$600 or send it in to Zippy, Charlie Wheatley, or Black bear for $250-$350 who are the most common here. They will do just as good if not better than a custom performance Dyno shop especially for light to moderate modified vehicles. We have guys with mail order tunes running 10's in the 1/4mile from the mail tunes. Just do a little research and you will find all the info you need... If you do a mail order tune you need to remove your PCM located just under your alternator next to your battery and is held on by a snap and two bolts holding the pins into the PCM, super easy to remove and mail it to your preferred tuner. Takes 1-2 weeks turn around. If you cant have your truck down for that long they can send you pre-programmed PCM for an additional fee and all you need to do is send them your core. Once they receive your old PCM they will reimburse you the additional fee. YOU WILL NOT BE DISAPPOINTED IN A CUSTOM TUNE!!!! Plus most tuners will give you free or small charge retunes for as long as you own the vehicle in case you ever want to add more go fast parts like superchargers, turbos, or even cams and heads. A stock truck will benefit from a tune but a truck with full exhaust and CAI and other bolt-on's like yours will benefit a lot more, so much you will think its a completely new vehicle. It will run smoother and be twice as responsive all while gaining 2-5mpg more... The more you add the more you will need a tune to get full potential... Here is what I would do if I were you
1st and foremost- If I were you I would get a tune setup. That way you can get the O2's deleted and unlock the true potential of these trucks...
2nd- I would get your truck scanned and find out which O2 is giving you the bad codes. If it is both the rears then I bet the cats are giving out or you are just unlucky and both went out. If the cats are shot I would just cut them out and put in a straight pipe to fill the gap or put in a new high flow cat if your state tests for it and still tune out the rear O2's as they are not needed for running anyways. If it is the front O2's going out you can use the rear O2's and swap them because the rear ones would not be needed when you get tuned. Make sure to have the bung holes welded shut or leave the old unplugged O2's in the holes so no exhaust leaks.
3rd- Id probably get a new exhaust system as that one seems pretty ratty and rusted in spots and may only continue to spring leaks. But if your on a budget at least do a leak test to make sure all the welds and pipes are sealed up as well as the headers. Holes or leaks near the engine/headers or near the O2's will make the O2's read inaccurate also throwing codes. Most exhaust shops can do that for you for a reasonable price, or you can just listen and feel around with your hand for air leaks trying not to burn yourself.
Finally- According to your pictures, the cat on the passenger side looks different than the driver side. It may be your problem as it looks like it maybe getting a bit warm due to the discoloration and extra rust which is an indication it could be plugged or not working correctly as compared to the other side. I am not there to look at it or do the test so I could be wrong but it is worth looking into. Another sign you can test at home to see if they may be broke or plugged is after they have been running for awhile they may even glow red hot or much hotter then the other. Try not to burn yourself lol. Also be sure to check especially around the hanger welds as I have seen them break or crack causing leaks. I no longer weld my hangers directly to the pipe due to the welds rusting out overtime. Instead I attach an exhaust clamp to the pipe where I want to weld and weld the hanger to that. No more cracked or bust welds on my pipe .
I do like those mufflers as I run similar Magnaflow dual mufflers and they sound beastly under acceleration. Anyways good luck and I hope this helps you find the problems... Get a tune