W126 are known to wander a bit at speed, especially in cross winds. Prior to replacing the steering box, ensure the steering box mounting bolts are tight, as they have a tendency to loosen with age. I had my steering "shock absorber" changed at 229K and it reduced vibration in the steering at speed quite a bit, and also appeared to help straight line tracking....however, I don't know if it was the new shock or re-tightening of the steering box bolts that helped out with the ability to track straight.