I can't say I have ever heard of a shock ever causing vibration at different speeds. Vibrations are caused by many things
but in an automobile it is caused by parts that rotate. Have you determined that it most definitely is coming from the front? Have the wheels been balanced lately? How many miles are on the car? Have you ever inspected the driveshaft flex disc? If the vibration starts at a certain speed and stays then the first thing I would look at would be the driveshaft and flex discs.