Andy
I am not a dif expert in any way shape or form but from knowing how they work and what is in there i find it hard to understand how a problem in this area would only causes a vibration at a certain speed. I would be interested to know what the dealer thinks is wrong with diff that would cause this type of vibration just for a learning curve for me as I have never heard of this before. But I have said that I am not an expert.
In my experience a vibration that comes in at certain speeds and under acceleration is most common to be wheel alignment, shock asorbers, wheel balance but as it apears to come from the rear of the vehicle I would grab the rear dirveshaft/propshaft near the joints give it a good hard shake around up down side to side and see if there is any movement. It should be almost as solid as a rock. If there is movement this might explain your problem as propshafts can also cause vibration problems as you get to or pass through certain speeds as this joint fails. The only other thing that I could think of is the bearings but as this has been done and there was no joy. The other bearing that could have damage is the input bearing to the diff itself or the out put bearing from the rear of the gear box. but the wiggle shake test should also show movement there as well. If all this fails I hope someone else has some ideas.
Adam
