I assume you took it up to speed that vibrates, like 85 or so, then disengaged the transmisison and driveline to put it into neutral at some point, correct? That would at least eliminate driveline and motor mount issues. Take it up to 100mph and disengage, see if it still shudders at 85 without driveline. With that eliminated, try doing the 85 mph ride again with driveline engaged and turn hard left and right like you are making an emergency lane change to see if the shuddering changes it's feel at 85 during the weight shifting left and right. If the shuddering stays, it's not suspension or weight transfer issues. (bad wheel bearings' noise will change pitch as the weight unloads off the bad bearing). So, if this is still an issue, at least you may have some more fodder for the cannon..... At least you will know if it is triggered by engine/driveline issues, versus axles, versus bent wheels, versus bad bearings, versus frame bent, or whatever it ends up to be....I would figure the dealership would do stuff like that to eliminate the problem since that's what they do for a living.... Just a thought.... eliminate big stuff, then get to the little stuff.
Hey, just thought of something else that might work.....why not contact your favorite dyno place, hook the car up to the dyno and see if the same problem occurs on a "smooth rode" that the dyno provides? If it shudders at 85, it's more of a controlled environment. Won't be able to make any turns obviously, but you could video the driveshaft, axles, and wheels and then view the video for imbalances/wobble..... frame by frame if need be.
