The rubber part of my motor mounts are cracked, well at least the top two. This is why I thought it might be that. J

Jay,
I had the tires balanced when I had them put on, like 3k miles ago and the shake is only noticed at 40-55 mph. At 120 the ride is as smooth as the inner thigh of a highschool cheerleader.
Knowing that, could balance still be the problem?