Sounds indeed like the clutch. That is basically the only thing that can slip, so I cannot think of anything else.

The whine sounds like the clutch release bearing.

The viscous center coupling cannot be the problem. Even if it would be completely worn out, it would still drive all 4 wheels. The only thing it will no longer do is transmit torque to wheels that still have grip when other wheels are spinning. With a worn viscous center coupling the car does not become RWD. It just feels more like RWD, because the rear gets 66% of the torque and will therefore spin the wheels before the front wheels will.