Need transfer case help on Ford Explorer

Can someone please tell me what the Transfer Case Motor does on a
2001 Ford Explorer sport 4 x 4 .

When we turn the switch to 4 wheel high or low nothing happens. Checked
the fuses...they are ok.

The reason I ask about the motor is because the outside of it is rusty. I thought
it might have gotten hot at some time.


Thanks,

Tony