The ones behind the cats rarely go bad. I'd say the two you are showing that are bad are the two closest to the front of the truck.

The only way I know how to test an O2 sensor is to take it out and mount it in a bench vise, then put your meter leads (set to dc volts) and measure between the ground (threads) and the wire in the connector. If you have more than one wire check the resistance between the wires and the two you get a resistance on are the preheater wires. The one you want to test is the other one.

You should get about 1 volt when you put a propane torch on the sensor, it should go away quickly when heat is removed. Then do a 2 minute test. Hold the heat on the sensor for two minutes, and watch the meter to see if your 1 volt is dropping out. If it is replace the sensor.

If you pass the 1 volt test and the 2 minute test, your sensor is good. Put some anti-seize on the threads and reinstall.

If you need new ones call merlin at St Charles Isuzu 1-800-727-8066. He'll give you stock parts at 15% off and no tax or shipping.

Hope this helps.