The Hummer H3.s will do this when in 4wd. Check and make sure that the 4wd buttons are not pushed. Push the one all the way to the left and then try. If it still does it, put it in neutral and push the right 4wd low button and listen to see if you hear it engage. If so, make sure it is stopped and in neutral and push the button all the way to the left and it should disengage. Sometimes after doing this you might have to put it in reverse and just back up 5 feet or so. If not, you might have to take it in to someone. Hope this helps.