Buspilot

My '99 4Runner's (5 spd/Cali) check engine light has come on so I took it down to the Auto Zone and they told me the engine code P0420 came up. The guy there told me that the forward O2 sensor needed to be replaced but I've read on this forum that this indicates that the rear O2 sensor is the bad one. The truck runs fine so I'm assuming it's the rear but I was hoping someone here might be able to shed some light on this for me before I hit the checkout button at Amazon for the wrong Denso sensor.

Thanks in advance for any help...