it could be many things, alignment, axles, wheel bearings, drive shaft etc etc.
Did you curb your wheel recently? a out of whack shape wheel that have a slight bend will make the car shake. It will shake from one side only where the jacked up wheel is. If it shakes all together, then this condition is out.
Does it make a weird noise, more like a helicopter noise. Does the noise gets louder as the car gets faster?
Does the car stop shaking when you hit the brakes?