I recently changed the ball joints on a W124. It too had the spooky steering on the highway - he had it checked by about 8 shops including a benz shop - who all checked it with wheel hanging - the correct way is to compress the spring (jack under control arm) and then lever the joint to check play.

For any odd reason you find the boot is ripped on the ball joint - immediately purchase a new joint, its far cheaper than paying tow, bodywork, upper strut mount and labor.

btw - the ball joints I changed out were really bad, one was sloppy and one was nearly siezed (rusted in place) car was a 2.8 300E.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)