Bobby G

I have a 2005 Yukon. For some reason the truck will just go into neutral while I'm driving on the xway, backing out of my driveway, or leaving a stop light. I then have to press the AWD button on my dashboard to get it going again. When I do, I hear a slight grinding noise and then a loud "thump"... When it goes into neutral, the 2WD light is still lit.... Once I'm going again I can push the 2WD button and it accepts it.. Any thoughts???