If they put the car on a lift and can turn one of the rear wheel with their hands while the other stays stationary, the diff is certainly gone, there cannot be an argument. My dealer, Tyson's Porsche in VA, has been awesome for the last 6 years, and they did the test without me telling them how to or had any arguments over the condition of the diff
The only thing we do not know is the condition of the ramps right now, we'll see sometime next week.