Easy way to check. Run it up to 70mph, vibration at 60-65mph, clutch it. Allow motor to return to idle. As you slow down through your vibration range, if it is tires it will still have the vibration as you slow through your target speed.

If no vibration encountered, it's engine, drivetrain related.

I had a similar problem with mine. Everybody blamed tires but I got no vibration while performing test as outlined above so I didn't believe them.

Turns out, for me anyway, the plastic/nylon pad from the primary tensioner had come apart with a piece embedding itself in the chain. I had a harmonic vibration at the speeds you mention that would put my legs to sleep from the knees down. Miserable.

 

Man that sounds like what i'm experincing I will definitly do that little check out and even pull the inspection cover to see if my wear pad is goofed up , or my chain needs tightining .

Thanks for all the input, I especially like the one about the rubber band on my pants, I would have never found that one....