Just a little update. It didn't take long for me to end up getting both codes repeatedly - P0420 and P0430 - so both sides were throwing codes. I replaced the front sensors first (husband insisted it must be fronts, although my money was on rears), and after a few months there was no change and I still got the codes. Then I replaced both rear sensors, and the codes are now gone - Yay! Don't need new cats, which I'm thrilled about. Truck now has 161,000km on it, or 100,000 miles.
2003 Tundra V8 4x4 Access Cab TRD Offroad Edition, 157,000km
2004 Mazda RX-8 GT, Fully Loaded with Nav, 101,000km