2003 Chevrolet Silverado drivetrain & axles problems

I purchased a new chevrolet silverado truck on 8/15/2003. I drove it 15,000 miles and the transmission went out and was replaced. On another trip returning from denver to durango I was driving on a mountain road when the back wheels suddenly, without warning locked up. This caused the truck to go into an uncontrolled skid and I crashed into the side of the mountain. The truck bounced back into the road and almost forced other cars off the highway. The opposite side of the highway was very steep with no guardrail.

Vehicle seems to go into 4-wheel drive by itself while driving . This has happened to me three times already. I want to switch dealers but GM told me no I have to stay with the dealer who worked on the vehicle. As this happens the vehicle slows down considerably causing the seat belt to lock and hold me back. Also at times the vehicle just slows down while at highway speeds. The check 4 wheel drive light comes on and the push buttons on the panel do not operate.