I have also been fighting a W126 front end vibration problem for about 3 1/2 years. I have owned 3 W126 cars over the years and my current car an '88 300 SEL w/172k miles is the only vehicle that has chronic vibration problems. I have gone through two sets of Michelins, one set of wheels, many many balances (including road force). Replaced the typical front end items, tie rods, track rod mounts, u/l control arm bushings, idler arm bushings and brake discs. Unrelated to the vibration issue, I've also replaced the f/r subframe mounts, differential mount, trans mount, l/r motor mounts, f/r flex discs and a steering box. My car drives straight, does not wander and handles great on curved roads. The vibration issue was first noticeable from 65-70 mph and got progressively worse from there. After the above mentioned parts replacements it is mostly now barely noticeable at about 70 mph but gets worse up to about 85 mph, from 90+ mph the car is smooth as glass. However, sometimes the vibration comes on strong at 70 mph and sometimes seems more noticeable on sudden highway speed deceleration. Ocassionally(but not always) w/hard braking from highway speeds a moderate to severe vibration is felt and l/r shaking is experienced at the steering wheel and pulsing can be felt at the brake pedal (not like an ABS pulse) but more like a warped rotor pulse. My current theory is that one or both of the steering knuckle/spindle assemblies is defective. My plan is to replace both along with wheel bearings and rotors.