I have a 2003 Ford Focus ZXE which shakes pretty badly between 35-55 miles per hour. Why?

I have a 2003 Ford Focus ZXE which shakes pretty badly between 35-55 miles per hour. No one can tell me why. One person said it has to do with how my alignment is off/my tires, but then why is it ONLY always happening between 35-55 mph? Any ideas would be greatly appreciated.

Get all the wheels balanced. If the tires are quite old (over 6 years), replace them. Get an alignment too.

Usually, improper wheel alignment won't cause a shake or vibration. Shakes and shimmies are caused by something out of balance. If the shake is like a wobble in the steering wheel, there's one or more wheels out of balance. If the shake is like a heavy vibration, the cause is something in the drive train like a bad universal joint or constant velocity joint. Bad wheel bearings can cause a vibration also. Any vibration will be more noticeable at certain speeds depending on the mass of the part that vibrates. What ever is shaking is doing it at all speeds, you just can't always feel it.