[Prematuregrey]

I would lean toward something being wrong between the the tail shaft of the tranny and the pinion of the axle. Drive shaft balancing weight is possible. I once had a violent shake on my 4Runner at speed and it turned out to be the nut retaining the yoke to the transmission output shaft had loosened up. Never had another issue after just re-torquing it. It could be a separated tire but it would be more continuous throughout all speeds, same with a wheel weight. As for it being a balancing weight, it would have to have been a big ass weight to shake as violently as the OP stated. Try rotating your tires front to rear to see if it migrates. That will eliminate tires/wheels.