Hi I am Ken from Ohio. Have a 2006 GMC Envoy. In 2wheel drive, waiting for a light to go green, I experienced a problem where I could not go forward nor go backward after I received a green light. I tried putting transmission into park and tried again to put in reverse and forward. Did this several times and still could not move. I then put into 4W drive and back in 2W drive and was able to move. No problems since. What may be the cause of this? Is it slipping somehow in 4W by itself?