I see you adjusted the steering box.

I've had the same problem on two MBs, where the "adjustment" went too far, and the box was the source of the resistance.

Back off the adjustment nut slightly and see if the return improves. Otherwise, the ball joints are a good bet....

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles