I don't think O2 sensors, but what I read. Sounds like a fuel trim problem, probably a too lean condition. Unmetered airflow. If it was just the rears, then the catalyst would be suspect. Usually the fronts are fuel trim code related. Not all the time mind you. You can have a bad catalyst, and then the codes would be performance/efficiency related, but without knowing what the codes are, I'd lean towards fuel trim. O2 sensors aren't hard to do though, O2 sensor crowsfoot are 22mm I think??