It is a harmonic situation, rather than an out-of-balance condition. It's present in earlier builds (14 and 15) primarily. I've run into a handful in my part of the business. Seems to be a stacking of normal resonances which come into phase around 70 mph. In the ones I have dealt with. Loosinging all engine, trans and diff/axle mounts along with exhaust and body mounts. Moving them around and "settling" these interfaces corrects it. This, of course, is after the normal vibration analysis is performed. Every dealer has an EVA, but just like everything else, the skill of the tech is all important.
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