I had a '15 RCSB and now have a '16 CC/6.4'. I developed highway vibration as my original GY tires got some mileage on them. It's been my experience if it is a balance problem, vibration starts to occur at 45mph and continue up to 75mph. Usually above that it will smooth out somewhat.
On my '15, I put almost 53,000 miles on it in the 19 months I owned it. Mostly highway driving. When it was new,didn't ever notice any vibration at highway speed. I'd say in the last 20,000 miles or so, vibration was pretty annoying at 65-75. At times if I went over that, vibration would get more pronounced.
When last November rolled around, I put my spare set of wheels with Cooper ATW tires on. Vibration was gone.
I probably put around 47,000 miles on the GYs. Tread wasn't wore out, but for some reason, they didn't seem "round" anymore. Also in that period of ownership, I had to replace 2 of the original GYs. 1 developed a high spot that I noticed while doing a tire rotation. I scrapped that 1. Another got a 1" break between the ribs and I scrapped that tire too.
On my new '16 CC, the new GYs don't have any vibration. Currently I'm running the Coopers. I feel that the GYs are not a high quality tire and if you have the opportunity, try a different set of tires.