I've got a 2006 Yukon XL, 5.3L/4L60E, 65K miles. Yesterday on the highway it suddenly shot the revs up. I was able to get off the road and drive it slowly home. It will shift into 1st and 2nd fine (as well as reverse), but won't shift up to 3rd. It acts like it's trying to shift to 3rd, but...