Fixing error code 137/177

[VTRangeuh]

Check engine light came on with stored codes 177 and 542. Tested running and got 177 I'm gonna replace the whole fuel sender assembly, but I'm not quite sure where the o2 sensor is to be replaced. I was under the impression there were 3 (based on a YouTube video) but I can't find the one downstream of the cat.

I have a 1993 3.0 standard cab short bed. Where is the 137/177 O2 sensor located and what's the easiest way to get er out.

Thanks