Transmission mount. If something feels like someone is kicking the floorboard, that's the first thing I'd check. It's either loose or worn out.
Does it do it in a straight line and just when shifting? If so, I'd really suspect the transmission mount between the trans and the rear part of the subframe.
Or does it do it when turning? If turning, could be tie rod ends or ball joints. Mine (manual trans though) transmitted a loud "thunk!" under the floorboard going around turns and hitting a bump while turning and ball joints and tie rod ends fixed it.