I take it the way to check and see if the steering coupler is bad is to move the steering wheel left and right while watching the front wheels. If the steering wheel play noted and wheels not moving then "bingo" it is the coupler.
If the subframe bushings are at fault then the wheels will move but the car's subframe will float left and right while driving. I am hoping I am correct.
My 450 SL is going through the same problem and I am going to check it.