Thanks very much for your suggestions. A software update did come to my mind as a potential resolution, also.

I would think that a faulty sensor would throw a code and the check engine light would come on, but maybe not?

I will be sure to mention the engine power issue to them. I also find that it seems to vibrate when idling in gear, which I'm not sure is normal, either. I understand that the Xterra isn't a Mercedes Benz, so it might not be refined to the point of no engine/drivetrain vibration, but in this day and age, I wouldn't expect a new car or truck to vibrate when idling, either. i was wondering if that might be another symptom of the issue. I could be off on this one, though. Maybe vibration at idle is normal for the Xterra. I'll add that to the questions:

Does anyone else with a recent model year Xterra experience engine/drivetrain vibration when idling?