Adam'14

Cyclic vibration in both seats at 63 mph and above, also in neutral. Vibration lasts 1 second, then smooth 1 second, then repeats incessantly. GM has replaced rear wheels, differential, both half shafts, and other whatever. GM has had my vette since 10-30-17, and won't give me answer as to how long they'll keep trying before they admit it it unfixable. I let the 2 year lemon law expire, expecting an easy fix last year.
2nd question, does anyone know how to email Mary Barra? Saw in this forum someone got email to her.