Ford Repair: 1997 F150 (small 8), probable causes, autozone

Question
Truck has lost MPG and knocks/pings under low RPM and load (going up hill).  Finally threw a code (took to autozone and it said running too lean in bank 2 - something like that).  Idles fine and generally runs ok.  I'm thinking its the driver side O2 sensor as the truck has 98k and I don't think O2s have been replaced.  Is this the place to start or could it be something else.  Don't want to waste the money if something else might be the cause.  New wires, plugs, fuel filter in the last 12 months. Thanks

Answer
Probable causes for pinging,
1. igntion timing off
2. egr valve bad.
I would have the vehicle serviced, at your garage. If you had a check engine lite, then the mechanic would tell you what sensor is bad. I wouldnt change the 02 sensor unless the code comes up..