My 2007 Suburban seems to run fine in 2WD, but when I put it in Auto (not 4hi or 4low) it shifts into 4WD unexpectedly, on dry pavement or in parking lots pulling in or out of spots. Then I get the Service Traction Control and Stabiltrak off warnings, after driving straight, long enough to have the transmission shift (it shifts hard, with a loud clunk) then the alarms go away and the car drives fine. Do I have a transmission problem, or is it something to do with the Auto 4WD controls?