1998 Ford Explorer O2 Sensor replacement, Bank 2, Number 1

It's right at the input to the converter on the drivers side.  Comes off with a 7/8" wrench, or use the special O2 socket you can buy (I didn't need it).  Two important notes when doing this ...

1. They say to make sure you put anti ceasing compound on the threads when re-installing, but mine came with the compound ALREADY ON the threads (so I spent a few dollars for no reason).
2. The connector is behind the engine, up against the fire wall.  You remove the drivers side front wheel plus the curtain behind the wheel.  Then, you can reach in and remove the connector.  This last one took me an hour of attempts and 'googling' to find.

Now, no more check engine light.