Did you ever find a solution on this? I too am having a similar problem. Under acceleration it is nice a smooth. At cruising or deceleration (especially between 30-40mph) the car vibrates pretty rough. Lug nuts checked first... I thought it was maybe out of alignment... I just had new tires put on, balanced, and got an alignment so I know its not the wheels because the problem persists. I also will have to check the motor mounts. Please post if you found a solution.

It is the path of least resistance that makes rivers and men crooked