[Jerry Clayton]

Shaking at a specific (repeatable) speed range can be air buffetting---stalling around the bulges of the body---

It can maybe be found where the body is in trouble by putting several strips of brightly contrasting to paint color strips of yarn on the body and photographing at different speeds from a drive by point or another vehicle(must be far enough away to not cause a draft)

You also can try driving in close formation with another vehicle(van, suv, semi,bus) and see if it changes---