The more I think about your issue...the half turn of pinion slop...it's just kinda vague. If the backlash setting of the gears opened up a tremendous amount that could do that. You say the pinion bearing preload seems ok(no vertical play in the pinion). No in/out play in the pinion?

What ratio and what gears are they?


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01 Taco xtracab V6 auto TRD white
mostly stock
88 Runner/V6/auto/31"/5.29gear