Do those bad vibrations appear or intensify when you apply the brakes? If so, there's a strong possibility that your car is tooling about with a warped brake rotor, or rotors.
The rotor is the shiny, silver disc-shaped component on vehicles with a disc brake system. The rotor can get bent out of shape due to heavy wear and tear -- basically, overheating from more stopping than that particular rotor can handle. Instead of being uniformly flat all the way across, a deformed rotor is raised or lowered on part of its surface. The calipers and brake pads, which squeeze the brake rotors to make the car stop, can't get an even grip on a warped rotor. Hence, vibration.
If you're not handy with a wrench, it's a good idea to see a brake specialist who can tell you the condition of your vehicle's rotors or brake drums (on cars with rear drum brakes).