What do your front hubs look like? Do they have a dial in the middle or AUTO on the outside cover?

I don't think you have the hub style they used in the newer Isuzus. If you are seeing a "gear" of any kind in the hub, then you do not have the style described above.

Do you have something mounted on the axle tube between the differential and the driver's side hub? Do you have a button on your dash for 4x4, or just a lever on the floor?

Michael