Mystery problem with my 2012 Yukon Denali XL 103k miles - Symptoms have changed daily in the course of a week. The first symptom was a sensation that the back tires were locking up while driving and a loud noise only lasting a moment. This happened several times. Then there were a few instances of RPM revving with a loud hooting sound for a quick moment. Next, it started making a clunk sound when moving from reverse to drive. Finally, yesterday when leaving a parking lot it suddenly slammed on its brakes while I was driving. It happened continuously and I had to have it towed to a buddy's shop. There are no codes. He determined the rear is braking intermittently while the vehicle is accelerating. He does not suspect differential as the rear wheels turn with ease when he manipulates them. Any ideas?