If it only happens downhill engine braking, and riding the brakes stops it its simply a design flaw.
If not i would take it back to dealer, could be something else, hard to diagnose unless i could actually test drive it myself.
Hmmm only thing i can think of is that if the rear driveshaft doesnt have a carrier bearing part way down it could be binding when off throttle, downhill under load.
Other thought if you are decending steep and engine braking without dragging or using the actual brakes it can be the secondary loading up and banging around.
Do a test run on the second point see whats up, if braking helps just brake.