Sure-grip problem...

Ok so my cone-type sure-grip seems to be giving me some issues... When the car is jacked up and I spin the driveshaft both wheels spin (as they should) but when I manually turn 1 wheel, the other wheel spins in the opposite direction! (This happens when the car is in neutral or park and the driveshaft doesnt spin) Also when I turn the wheel back and forth the other wheel seems to slip some when changing directions... Any suggestions please? Is the rear toast or just some type of adjustment I'm missing? (Also makes a light humming sound when driving)