2004 GMC C2500 sensor codes

What kind of tests set? That is probably your problem, I suggest you take it somewhere and have them put a real meter (scanner) on it that will tell you which one it is, and which side its on and if it's a forward or backward one, keep in mind O2 sensors are only good for about 60,000 mile, now to answer your next question, yes a vacuum leak or a fuel pump issue will cause your O2 sensors to malfunction, but you are not describing any other problems you are experiencing with your vehicle, so have it re-diagnosed and let me know what happened.