there are many things that can cause a vehicle to shake.
out of round tires.
out of balance tires
bad inner CV joint
bad harmoinc balancer
the more information you can give me as to when it happens, where do you feel it, what speed, what rpm, what gear on the trans, at idle, when it's rev'ed up, when under hard acceleration, decel, cornering, starting from a stop, when does it stop shaking, does the shake match the speed you are traveling (goes faster if your car goes faster), does it match the engine rpm's (like when the engine rpm's go up and then go back down when it shifts gears).
Do you feel it in the engine? In the steering wheel? from the front? from the rear? From the left or right side?