It is rare when a vibration at highway speed is not tire related.
On the ASE suspension and steering test, there are several questions that ensure that the technician knows this. If it's so common that it is on that test, it is COMMON!
The sad thing is, they ensure that techs know that this vibration source is common, but they have no questions surrounding correct balancing procedures. There should DEFINITELY be something on the test to see that they know how important dynamic balance is to this equation.