4x4 engages in all (Auto, 4 High, and 4 Low) after driving for about 30 seconds in any of the 3 drives, the service 4x4 light comes on/shift button blinks and it will not shift out of 4 wheel drive. I have to stop, turn off ignition, then when i start it back up it will go back into 2 wheel drive. Any idea on what the problem is?