Transer Case; 2003 Silverado, Z71/4x4 (NP246)

The other day I was cruising about 35mph when all of a sudden the transfer case (I believe) started cluncking, and the my truck started slowing down. Winding down to be more specific. It felt like what happens when you go from "D" (drive) to L1. The clunking was speed related. Even when I slowed to 1mph the clunking was still there. It also did it in reverse. Desprite to get home I continued on, slowing increasing my speed. It eventually went away. But, it has happened a hand full of times again since then. AAMCO couldn't "duplicate customers concern." So, no help there. Another time, after driving on the highway for about 30 minutes, I came to a stop and as soon as I stopped the truck lundged forward and it sounded like the transfer case shifted gears. This all sucks cuz AAMCO doesn't want to take it apart until they can duplicate the problem. Good I guess, but the whole thing is driving me crazy!!