i know that you have been to the dealer and i'm not trying to second guess them. but....sounds to me like you could have a vacuum leak. it's very seldom to have both sensors go bad, esp. the rear one. i'd check all the vacuum lines first. i had the same experience. i found a bad rubber elbow by the intake manifold and a disconnected one on the mail intake hose (after the maf sensor, just under the turborcharger. bay13 could help you locate the more common places where leaks could occur.
good luck.