GLK350 4matic thump/thud/thunk when braking
 

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?

It could be the torque converter clutch dragging/being slow to let go when coming to a stop....but based on the varying speeds it can happen I'd suspect brake rotors

Also not feeling it in the steering wheel doesn't seem to mean anything on the GLK....I had a similar issue a while back and it was warped front rotors with only 7k miles on them, replaced them again and its been perfect ever since. The shudder on mine was in the body/whole car, next to nothing in the steering.

Has this vehicle had the transmission fluid/filter serviced at regular intervals (no more than 60k miles)?

These clunking issues are annoyingly difficult to diagnose, but I think you also need to consider the axles.

If I had to play Parts Horseshoes I would start with brake bands and a re-torquing of caliper bolts, since they will be due soon anyways. Then the most likely suspect, brakes, is ruled out.
Tranny mount would be next, but i’m skeptical.

Is the impulse of the shuddering at the same frequency or does it change with deceleration?

Phil Forrest