When I engage four wheel drive only the front left wheel is spinning. I look at the axle inside the wheel on the right side and it looks like the shaft is spinning but not the wheel. Any ideas? It's a 1997 Polaris sportsman 400 4x4 2 stroke.

 

If you haven't changed hub fluid,you might try that first. If it doesn't help check that you have both small awd connectors plugged in that go to the struts. Located under the front panel. Just trace the strut wiring up to the connectors. If none of this helps you may need to check out the mechanical parts in the hub. Dirt Cheap Yamaha, Honda, Arctic Cat & Polaris OEM Parts & Accessories – Cycle Parts Warehouse