Sorry, no clue. Does it only happen when the steering is turned? Generally it's not good to hold the power steering at the bumpstop for extended times since it puts strain on various drivetrain parts but if it occurs even when the steering angle isn't as severe, it is likely another problem.
How quickly do you shift from D to R? Does the shaking only occur once the transmission lever is in R? I also assume that the vehicle is completely stationary before changing gears, right?