This has been happening to my 97 Discovery for a while but its got pretty bad lately, on the freeway when going over an overpass or just merging lanes if the road gets slightly bumpy the car will start vibrating insanely until the steering is straightened out.I changed my steering damper, and front shocks, recently ihad the steering box rebuilt. any ideas?

 

Maybea bad tie rod end?

 

Check the bushings and more importantly have some set the pre load on your swivels joints.
Also, have you checked your u_joints?
Mike