AWD not working

I purchased a used 2004 AWD last year and the AWD doesn't work. I did research and discovered that it is a system that uses a viscous coupling and it doesn't work. I have videoed the front slipping and no power to the rear. I have taken to service but they say they place on the rack and all 4 wheels spin. I explained they should try to stop the rear wheels and see what happens. To me if you can stop the rear wheels by holding them, the system is not working. The rear differential was replaced due to noise but it still doesn't work. I can't find the part for the viscous coupling. A main factor on purchasing this vehicle is the AWD but it doesn't work. Any ideas on what I should do?