Ford Bronco 4 wheel drive noise?

On a full size 1984 Ford Bronco with auto , when you move 4 wheel drive selector on floor from 2 wheel to 4 wheel high all is fine. But when you move selector to neutral for the transfer case and then try to go to 4 low or back to 4 high, the differential makes grinding noise. This is even when stopped and the in neutral also. Any ideas.