Johnbt,
Shaking at a specific speed is generally a tire/wheel imbalance. It comes in at a certain speed and goes away again due to harmonics. To test it, make it vibrate on the highway, speed up to the top speed of the vibration or above it, then shift to Neutral. As you slow back through the vibration speed if it continues to vibrate it's likely the tires/wheels. Could be the driveshaft but that will usually cause a vibration that comes in and out a few times through speed range. Stop in at a decent tire shop that can Road Force Balance your tires. This is a machine that balances like a normal machine but it also has a roller that pushes a load against the tire. The reasoning behind this is that some tires have hard/soft spots and the differences will cause a vibration. In normal operation the tire is loaded. If it is balanced while unloaded then it is slightly false.
