2002 Yukon Transmission Problem

I purchased a 2002 Yukon in November 2004 with 48000 miles. This weekend, I heard a large "clunk", transmission started making strange noise, and I lost my reverse. The truck has 59000 miles on it now and I had the fluid changed at 55000 mi. The technician said the reaction shell was stripped and that the leading cause of this is changing from reverse into drive while the vehicle is still rolling backwards, or not coming to a complete stop before changing gears. He also said the "sun" gear was gone. Has anyone ever heard of this?