My boss has an '06 2wd Frontier (Automatic). He called me a bit ago with the following scenario:
It sputtered and then stalled on him when he was braking on his way home today. He started it up again and it wouldn't go above 2000rpm and it stayed in 4th gear. He could put it in Park and Reverse, but as soon as it went back into Drive it would be in 4th.
the tranny has a fail safe limp mode that puts it in 4th in case of problems so you can limp home same goes for the rpm, in case of problem limp mode will limit the rpm. why this happened with no code is a very good question though....