Rear Diff Problem

Can anyone advise me as to how to diagnose a worn diff. When slowing down I can hear a pinking sound [sounds as though timing out on a petrol car] coming from the back end. I've lifted all 4 wheels off the ground and put screwdriver onto diff and listened to it, but can't hear it. I also noticed that the passenger side wheel doesn't turn as fast as the drivers side, is this normal on a 4 wheel drive. How much play would be allowed when turning the prop by hand before the wheels turn. Finally if the diff is dying anyone know of any especially cheap diffs going for grabs. Its a 89 classic with a 200tdi transplant with Auto box. Thanks in advance for any help.