1979 austin/leyland sherpa petrol o series engine. At about 20mph my van starts to shudder vibrate quite badly and gets progressively worse if I dare go at a higher speed. The thing is it only happens when the van is in drive. If I free wheel it disappears completely. The problem appeared all of a sudden like something had broken rather than progressively worn. It was so sudden I thought it was the quality of the road. It seems like a suspension problem like I'm driving over a continuous series of ribs in the road except the fact it goes when free wheeling rules out a suspension problem, no?
A failed engine mount could possibly cause this symptom.
You could get someone to watch the engine as you select drive and reverse while stationary with the brakes on. If the engine moves excessively then it is likely an engine mount. Make sure the spectator stands to the side of the vehicle, not in front of it.