check your u-joints and look for a missing wheel weight.
i ran into this problem a while ago started out with minor vibes in the same range. did some searching and found that its either bad u-joints, out of balance tires, and in my case out of phase drive shaft.
so i started with the tire balancing didnt help, then checked the u-joints *rear shaft both ends where pooched beyond recognition, front axle ujoints had no needle bearings left* so replaced all those *had the front tore apart anyway so did the front wheels bearings at the same time.* that left me with just enough vides to make seeing out the door mirrors hard so un bolted the rear driveshaft from the diff pinion and rotated it 180* and viola no vibes left. well except for when the speed needle is touching the right side of the odometer on the highway but thats expected when your RPM is maxed and your needle cant go any further to the right LOL