There does seem to be a lot of movement. I wonder if the bush is fitted correctly? I suspect that the cut-out section should be top & bottom, not front and back?
Even if the the bush is too soft, I am struggling to imagine what, specifically, could make it cause the effect you describe. Driving at a steady speed, there is very little force acting upon that bush. There's enough engine torque to offset the aerodynamic drag, but the torque reaction on the front bushes is not large (because of the length of the trailing arm), plus it is a constant, steady force, not something that would tend to start a rocking movement.
Try driving along and, when the rocking effect is happening, dip the clutch.
If it carries on rocking when coasting, then I can't see how the bush is the cause because there is virtually no forces acting on it when coasting.
If the rocking stops immediately, then it is related to engine torque. I would check the engine mounts... perhaps some wierd 'resonance' between soft/broken engine mounts and the rear bushes? Although I'm doubting that, to be honest!
John
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