Hello guys, this is not really an performance ford but I hope you can point me in the right direction.

My 81 Granada 2.3 4 speed with 76.000km vibrates while driving past 50mph.
I tought it was the rubber in the middle of the propshaft, checked it, had some play so I changed it. Now it's even worse, maybe because I stiffened that part up. U-joints are good. When I took the propshaft away from the car I marked it's position to be sure it would go back as it was.

Where to look?