Don't rule out the possibility that it could be your tires. Sometimes tires split from the inside...and that would cause the same effect. It may drive fine until you reach 25-30 mph then it will start shaking violently. Have your tires checked first.....it's the cheapest....then go from there.
It's happened to me.....
Knowing what make, model and year of your vehicle, would be most helpful.
Regardless of that, if your front end shakes at a specific speed, it will almost always be a tire or a tire/wheel combination problem.
If it shakes, starting at low speed and continues through high speed, it would more likely be a steering or suspension problem.
Wheels out of balance can also cause this type problem.
It could be tires off balanced, like Karlton said cracks, motor mounts can also cause a car to shake/vibrate it could be many things and like all situations with cars you'll probably find the problem when looking for something else.