A stupidly easy fix to a complicated problem: These have been mentioned before, but I think they deserve their own thread. The OBD II diagnostics require the ECM to monitor the performance of the cats and set the "Service Engine Soon" light if they fail. So, if your high flow cats (or no cats) are setting the SES light, you can have the rear sensors programmed out of the ECM, but an easier solution is to go to
http://www.o2sensorsimulator.com/
For a whopping $25.00 you can buy a little electonic device that cures the problem. It is about 2 inches square and a half inch thick. You simply disconnect the sensor lead from your O-2 sensor (either one or both rears) and wire this in instead. It outputs a signal that varies from .18 to .25 volts and the ECM thinks the cats are working just fine. You need to leave the original O-2 sensors powered, even if they are no longer in the exhaust stream because the ECM will also set the SES light if there is no voltage load to the O-2 sensors. Oh, don't look for California Air Resources Board approval number on them......
FYI, the front sensors actually control the running mixture for the engine and are essential in the system.