I think I felt similar today going along the freeway.
I first felt the pedal vibrate then the rear felt as if it was shuddering slightly coming more so from the rear right..
I did it as I accelerated so I eased off, and floored it again to see if it would do it again and it did.. And it hasn't done it again.
I was thinking perhaps it was the road, like ripples or something, but I'm not sure..
I hope its nothing too serious.
It has been checked recently and front bushes need doing I think..