Did you try clearing the DTC's first? The problem arises from removing the cats, and the rear set of o2 sensors are there just to check that the cats are there and working and those are the ones that are giving you the code. The front set measures air-fuel ratio and sends the info back to the pcm to adjust said ratio on the fly while driving, leave them alone the are fine.
There are only 3 ways I know of to get rid of the codes, 1st, get some spark plug extenders, they move the o2 sensors out of the direct stream of exhaust gas and it fools them into thinking the cats are still there. Sometimes they work, sometimes they don't. 2nd, install some MIL eliminators. They are basically a resistor that installs inline between the o2 sensors and the harness going back to the computer. They work by intercepting the signal and modifying it so the pcm thinks the cats are still there, They are cheap and work well, until they burn out then the code comes back. 3rd, and the correct way and permanent way is to have a tuner write a tune that turns off the rear o2 sensors, you will never get another o2 code again, as long as you use that specific tune. Now be warned, option 3 is technically illegal, most competent tuners will not write that kind of tune, and the ones that will want to see the car to verify the cats are still there. About 1 in 10 vehicles that get long tubes throw a code anyway even with cats still there and the tuners know that so they will write a tune to turn off the rear o2's, but if you remove the cats, it's illegal to run said vehicle on the street, so there-for most will not write you a tune. If you find someone who will, get it and don't look back.
My advice, get some spark plug extensions and try them first, if they work great, you're done. If the extensions don't try the MIL eliminators, just be prepared to replace them periodically, they are meant as a temporary solution until a more permanent fix can be acquired. You could always put the cats back on and solve your problem and be legal at the same time.
