You are not the only one, there are issues of the 535 xdrive with weird vibration felt in the front seats from the rear wheels on highways between similar speeds (70-80 mph) as yours that cannot be resolved with new rims, tires, proper road force balancing. It is a design or manufacturing flaw. You can feel small vibes in the seat back and in the headrest that goes away below 70 and above 80 mph or so.
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