I suggest you remove the steering box and bench-test it. If your mechanic's adjustment made it worse, that suggests he did not know how to do it on the car. The factory bench procedure should then be followed. With the box off, you can also test for slop in the steering column.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7