Damaged suspension?
Damaged linkage?
PS fluid low?
Steering wheel and steering column matchmarks aligned?
Steering column coupler good?
PS pump output sufficient?
Tire hitting body/chassis?

If all the above checks out...
Maybe box is bad.

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