Do you have a tuner? My first bet is the mils wore out. They are basically a resistor to fool the system into thinking the cats are working... If you have a tuner (or know someone that will let you use their unmarried one for an hour or so) you can try turning the rear O2's off as an easy way to pinpoint the problem. There is also some good writeups on how to make your own mils, cost a few bucks and a trip to radio shack.
The artist formerly known as SteedaMatt.