1200rallycar,
Not trying to target you but from my understanding this is incorrect.
The reason is the relative radius which is used to determine the amplitude of the sinusoidal angular velocity is a function of uni operating angle. therefore if the front and rear uni's are not very closely aligned the do not completely cancel the wave out.
Wiki explains it better then what I can, see double Cardan Shaft for explination
Wiki
ssskiwi, Agreed, although there is a recommend maximum operating angle of most uni joints.
Anyway Ive given more then my 2c on this topic. I hope it helps old_school diagnose his issues.