O2 SENSOR REPLACEMENT 2000 z71 5.3

Check engine light has been coming off and on for a couple of months. Finally had the codes read and it is telling me to check bank 1 (o2) and something about the performance about my catalytic converter. My guess is that a bad o2 sensor is causing the code from the cat. converter. Parts house tells me to replace all sensors (4), that seems a little extreme. At about 70 bucks each, I decided to hold off.

My truck has 4 sensors, I was told that bank 1 means the front sensors. Any way to designate which front sensor is bad? Also, should I replace all sensors, or just do it one at a time? Any help would be appreciated