Maybe a steering component? Such as the stabilizer or a tie rod end? Definitely worth looking into before the box. I could only imagine how much of a PIA, changing that out could be. Have you done any adjustments to the box? Maybe it's adjusted too tight (IIRC, there should be at least 1/2" of play) I haven't read of any cases of failed steering boxes, here on the forum.

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Sam

84 300SD 350K+ miles ( Blue Belle )