Check all your spark plugs, spark plug wires and fuel injectors. Run some STP full engine cleaner in there (the kind in the bigger grey bottle) just for the hell of it too.
My car shakes around 40-55mph with gas applied lightly, so I keep it in 3rd to keep the RPM'S up so it doesn't shake until I get to 60mph, then I shift to D. I got five new fuel injectors, timing chain, water pump, radiator fan, and engine gasket and none of that fixed the problem. The mechanic I go to thinks the shaking is continuing because of the vacuum tubes or spark plug wires.
I don't know if that helps, but ever since my car started shaking, its been more sluggish.