Stuck in low?
You know that it's programmed not to shift in and out of low range unless it's in neutral and your foot is on the brake, right? It won't shift in/out of low range in park. and if it's in low range, it's in 4x4.
If that doesn't work, make sure the brake sensor works (brake lights come on) and the computer knows when the car is in neutral (i imagine there's an MLPS but i have no idea how you'd test it).