mazda 3 2005 steering wheel vibration at 50mph range

Have you checked your tires lately? Air pressure low, out of balance, alignment problems, all of these can cause your vibration. Does the vibration go away or get worse the faster you go?

You didn't say what things you've already checked but the previous answer had a lot of good information. Start with the simple things first before you start diagnosing drive shaft linkage problems.