I also have autostick, but I just had the differential replaced on my 2004, as many others have done. The usual symptom is a whining sound from the back when under throttle. Mine kicked in at about 40 mph and became very loud over 70. As soon as I backed off the throttle it would stop until I gave it even a little more gas. If that sounds like what you are hearing then it probably is the problem. You will find a number of posts on this forum addressing the same problem. Since the differential was replaced under warranty the noise is completely gone. Good luck.
Dave Berry