I wouldn't replace the front one again especially if it's the 3x more expensive Air/Fuel ratio sensor that the newer CA emissions and 01-02 4Runner had. However it's very possible that your rear O2 sensor has quit, especially if it is original. Look into a replacement at
DENSO Auto Parts | Home and look up the specific part number. Normally Amazon has the lowest price and should only be $50-$60. Much much cheaper than a new cat and super easy to install assuming you don't have rust.
This is probably the most straightforward check engine light to address. It's either an exhaust leak at the cat and the rear O2 sees too much oxygen, the rear O2 sensor has failed or the cat has failed. Or if you're lucky like the guy who posted up lasted week, there's a 0.00001% chance it's a dirty MAF. Worth a shot cleaning it I suppose.