2001 Cadillac Escalade AWD & 2 wheel drive issues

My wifes escalade keeps droping out of 2 wheel hi into 4 wheel auto. What is causing this problem? It does it randomly, To fix it she pulls over, puts the car into neutral than pushes the 2 wheel button and it shifts back up and she goes again. It usually happens between stops when she turns the car off and on and she notices it when she starts driving.