Propshaft Vibration

[scrab]

I get vibration through the car which is worse at 50 mph. Above 60 it disappears. I've heard some of the replacement centre mount rubbers can cause vibration. If anyone has had experience of these poor quality mounts I'd like to know if it occurs at a particular speed or all the time.
I suspect the propshaft is out of balance but would be grateful for other opinions.

 

[scrab]

I've had the balance of the two props bolted together checked and been told they are OK. What I have noticed is that in the parts book the 1st UJ assembly (next to the gearbox is 90 degrees out of phase with the centre UJ. Is this correct as mine is in phase.