GLK350 4matic thump/thud/thunk when braking

[ur20v]

So I maintain an elderly woman's 2011 GLK350 4matic for her. She told me it's been shuddering under braking, and sure enough, it will often (but not always) shudder under braking. BUT - the shaking doesn't originate at the front wheels/brakes and it does not affect the steering wheel in the slightest. You feel the shuddering through the entire car (but still coming from the front of the car) when it does it. I checked the control arms, ball joints, and strut mounts today, everything is solid and in good shape. The calipers are firmly attached, the pads and rotors all in pretty good shape, save for a little grooving on the front rotor faces. Tires are brand new as of last week, decent Falken whatevers, and they were road force balanced. I checked the torque of the wheel bolts and everything was on spec. She asked me to drive it again today, and I noticed that I could cause a loud thump/thud/clunk if I jabbed the brakes at any speed. After verifying I could cause it at 35-55MPH, I drove through an empty parking lot at 10-15 MPH continually jabbing the pedal and causing the clunk each time. However, as hard as I tried, I could not replicate the thud while moving in reverse, even at speed. The motor mounts seem okay, as the engine doesn't torque upward while powerbraking, but the whole car will jump when the transmission is put into drive (but not reverse or any other selection). Could it be as simple as a transmission mount? Or is possibly something more sinister, like the transmission or transfer case/front differential? She's complained of another issue that I've experienced as well that *is* originating from the drive train, but it seems unrelated at present and I won't confuse the issue yet. Any ideas or suggestions?