This might sound a litle crazy but check your coolent over flow tank and see if the fluid is not the right color or even dark. I had my trans being hesident when changing from P to D and if it did not catch right away it would slam in to gear and shake the car. I took the car to the mechanic and had him check every thing out It turned out that the trans cooler that is in the Radiator had a leak causing the pressure to drop and did not show any change in the fluid level.
Once replaced the problem went away and has not came back (knock on wood)
93 Buick LeSabre (83K)
2000 Grandprix GT (63K mi Totaled at 76K on 8/25/6)
Green 1993 SE (No longer with us.)