Why does my car shake from the front to the back?

Have the tires and wheels checked and balanced.

Yes they were when they were put on

Think you misunderstood the answer Bob provided...he's saying you need to have the wheels checked for damage and/or rebalanced. It's possible you've lost a wheel weight that is causing the vibration or you may have suffered wheel damage to cause the vibration (i.e. - you hit a pothole and damaged the rim slightly). One thing I've noticed with these Buick wheels is they're pretty good about collecting mud on the inside edge and having that dry in them causing an out of balance condition - I sometimes have to spend a good 15 minutes with a pressure washer just to flush out the dried/caked on mud and then all is good with my wheel balance again. I've had other Buick owners report the same occurrence to me - they would drive someplace muddy such as on a wet gravel road and then a couple days later they notice that their vehicle has a vibration at highway speeds.