1991 Buick Century Vehicle vibrates or shimmys at 40 mph

I just purchased a 1991 Buick Century w/79000 miles for a daily driver. It has new tires and drives really nice except for a vibration that starts at 40 mph! I had the tires re-balanced, but the vibration is still there. They said they could not perform an alignment because the LF wheel bearing is bad. Could this be the cause of the vibration? The car drives perfectly straight, it does not pull to one side or the other. I also do not get any vibration through the steering wheel, the whole car just shimmys starting at 40 mph. I jacked the car up and tried to wiggle the wheel and it didn't seem to move when I pushed and pulled on the top and bottom of it. It did move a little when pushing and pulling on the left and right of the wheel, but it looked like the tie rod was also moving with the wheel. Is the hub assembly bad or could it be something else! A point in some direction would be greatly appreciated!