After reading through the posts here, I thought for sure it was the ball joints. Once I got the car up on jack stands and had someone turn the steering wheel back and forth, I could tell it was the steering damper strut.

The noise was actually coming from the center, not the knuckle ends. When I sprayed LPS2 lube horizontally into the cover tube, the creeking noise stopped. What a lucky break--$57 for the damper strut versus $180 per side for the ball joints.

This part went bad on my old 400E too, but it was making only one clunking sound when turning left, which was most evident at slow speeds. It turned out to be the damper strut knuckle on the passenger side that was completely shot.

I'll take a damper strut any day over lower control arms.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles