Finally had time to go for a decent drive on a quiet open road without too many potholes. On several long downhill stretches I put the car into neutral and coasted with the speed dropping slowly from about 100kph. The vibrations I have felt at different speeds under load could ALL be felt with the engine idling as the car slowly lost speed and all occurred at the same speed readings as when the car is under load: 85 - 75kph, 60 - 50kph and 40 - 35kph. 3 distinct speeds at which vibration is most noticeable.
To me that clearly says it has to be in the tailshaft or further back, ie the diff, rear axles etc and NOT in the motor or transmission.