Im planning on purchasing some of the infamous 94106's but my question is will i have to do anyhting with the O2 sensors? Are the O2 sensors even on the rear cats? Im planning on just replacing the rear cats with hiflows.

 

Cali emissions trucks have sensors in the middle of the rear cats. 49 state emissions have them after the cats. It should be a direct swap for the shop. Either way, the rear sensors need mounted after the new cats to read properly and keep the CEL off.