Sounds quite normal to me under heavy braking. Heavy braking in gear will result in slowing the engine very quickly against a huge rotating mass, with the engine fighting to idle/maintain the revs in a smooth manner. If it didnt do this, every time you lifted off throttle in gear, you would have a huge lurch forward with the sudden and drastic engine braking as opposed to the fairly smooth transition you should have.
If its a much more physical issue, where you can actually feel weight/mass shifting under heavy braking, then i would explore further, checking engine and gearbox mounts, front arm bushes, subframe etc.