Vibration at 70+ mph

[boomerang2120]

On my first real highway distance riding over this past week. I noticed the bike was nice and smooth as expected up till 70mph. At 70 it slowly starts to vibrate and increases with speed so much that by the time you hit 80 you feel like you're on a massage chair gone berserk. Slow down to 65-70 and the ride smooths back out. It is very predicable and consistent. Tried different gears, up and down hills, under power, and power backed off - it's not related to engine speed. It is related to actual speed of the bike.

Tires are the standard Dunlop's with maybe 500 miles on them.

I'm thinking it's just a wheel balancing issue, but thought I'd see what y'all have to say.

Muchas Gracias in advance!