I take it you have read both the Chilton and Haynes manual for your model year, right? You have also pulled any codes from the computer? If you haven't, do so now; this forum probably isn't the best forum to answer your question.
I am personally suspecting the rear sensor. If I were you, I wouldn't bother with further testing, and just replace both (assuming code or other tests don't indicate something else); if it is the sensor, chances are that if one is going bad, the other isn't too far behind.
Good luck!