If this is a continuous vibration while driving at highway speeds(55+) I would look into having your tires balanced like mentioned above and also have them perform a alignment. With a vehicle of this age, I would look for a way to buy into Lifetime alignments. I have had a program like this with firestone in the past.
The service bulletin for this issue always states that you should first look over tires and suspension for anything obvious that may be out of place. Like a bubble/flat spot/ or something sticking in the tire. Next would be alignment.
If the issue is not fixed, look into replacing lower control arm and bushings.
On another note, if you are experiencing the vibration only while braking. You must have the rotors turned down. Basically they take a paper thin slice off of both side of the rotor to ensure that it is smooth.
I hope this is helpful to you.
