2001 Renault Laguna body shudder

Hope someone can help me with this one my daughters laguna estate has developed what I can only describe as body shudder rather than vibration ,this happens at around 40 to 50 mph but only under load , as soon as you lift of the power slightly it stops . Sorry- more detail on the shudder the front of the vehicle feels like the engine has come loose moving the front from side to side and up and down, while this is happening the sterring wheel has no wobble or vibration .Have changed both engine and gearbox mountings plus top and bottom stabilizer bars ,all the wheels have been balanced there was a slight improvment after replacing the mountings , so any help or suggestions would be appreciated .