As I went to replace the O2 sensors, I found that one of the vehicle harness leads to the upstream sensor had been severed. I patched a new wire in, but will eventually want to replace the harness. Unfortunately, since the O2 sensor had been malfunctioning for thousands of miles at this point, it likely damaged the catalytic converters as well. I had Boulder Muffler do the work as I am between houses at the moment and all my tools are in storage. The whole process was fairly expensive, but the truck passed emissions with flying colors.