Have you checked the wheels for balance? Another possibility is a tyre "out-of-round"; easily checked while wheels are being balanced. A wheel can be balanced but not round.
Mine was shaking badly at 100-110 Km/hr Obviously that was particularly annoying given that it is the speed limit in most of Australia, and we live in the bush so my car is driven at that speed constantly. It turned out to be a front tyre (A/T pattern) that had worn out of balance. After a balance - still noisy but no longer shaky. It looks like these cars are fairly sensitive to wheel balance issues.