In order for your truck to tell which bank it is it has to have four 02 sensors or it could not verify which bank is lean if it did not and only had 2 sensors. I dont think its a high fuel pressure problem as that would effect both banks. It sure could be a bad 02 sensor but without taking readings on a scanner you will be guessing. Many throw an 02 sensor in because they think the code means the sensor is bad but it does not it just means the PCM sees a problem in that circuit as it could be any part of that circuit. If you have a vacum leak the PCM will adjust addaptive fuel and 02 readings to make up for it as long as its not to big of a vacum leak. Thats the thing about the electronic fuel injecton as with a small vacum leak the PCM can add fuel and keep the vehicle running good so if the check eng lite did not come on the cust may not know there is a problem but he would have much worse fuel milage. One thing to remember is the 02 sensor is just a fine tuner as by itself is can not control alot of fuel adjustment. What happens is when the eng needs more fuel then the 02 can adjust then the addaptive fuel memory takes over and starts changing the addaptive fuel cells to add fuel and once it changes enough to make the 02 start moving again then the 02 fine tunes the final adjustment. Also the downstream 02 sensors are more important then many think as it helps adjust the upstream 02 sensors switching goal which is the point it switches form lean to rich. The normal switching point is about .5 but it can tell by the downstream 02 sensor if its on the rich or lean side as the down stream looks at oxygen in the exh after the cat. It can move the point at which the upstream 02 decides to switch from lean to rich to help fine tune the fuel mixture. I have seen some with the switching point down at .42 and up at .64 and so on. Of course the downstream 02 also has its switching speed and amount checked by the PCM to determine how well the cat is. So the downstream 02 is more important then some think.
Your best bet of course is to have it scanned first to determine the problem but you can do like many and throw an upstream 02 in it and you may get lucky and fix it as that fixes that problem many times. Also Bank 1 means the bank the #1 cyl is on. Ron