My 92-960 has a CV joint at the diff. I also have a vibration that I can not get rid of. I took the shaft off and took it to some experienced guys and they could not find anything wrong with it. I had no luck shimming the center support. But I did get lucky when I reinstalled the drive shaft. Somehow I lucked out and installed it in such a way that the vibration is about 90% better. No shims just plain dumb luck (there is some sideways play possible on the center support). This leads me to believe that the vibration in my case is liked to an out of wack geometry on the shaft coupled with the soft rubber bushing in the center which makes the shaft dance under load. I spent a significant amount of my time tinkering with it and eventually gave up. I chalked it up to an aging car. Let us know if you have any luck.

92-960 , 195 000 km