Autozone or Oriley's should be able to tell you the code for free.
There is a TSB, not a recall out on the rear O2 sensors. Requires a reprogram of the ecu, and a replacement if that does not clear the code.
There are 4 O2 sensors as stated above. Nissan has the engine reversed when talking about banks, so be aware. Normally bank one would be driverside, but that is bank 2 to them. Bank one is passenger side.
There is an O2 sensor before the cat and after the cat.
You can change it yourself, but they are a pain to get to. Especially the 4wd ones.