If its kind of a tuning fork vibration at those speeds, I have it as well. It goes away as I speed up. I have about the same mileage and the flex disks were never replaced. I plan to change mine since some people say its a maintenance item anyway.

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1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma