vibration at certain speeds can be mostly caused by worn CVJs or badly repaired CVJs, Lower arm bushings also play a major role in vibration, the bushings should be very solid and only tightened when the wheels are in normal driving height, Vibration can also be caused by loose steering rack on the bulkhead firewall, it may need new bushings.SImilarly the rear also has panhard rods to control the beam movement - check all the bushings for wear and tear, if any are loose or torn - they need to be replaced.