Make sure trunion is tight (under driver's side header) and that its bracket's welds to frame are still fully secure. If so, you might then also check the 2 allens on underside of shifter shaft that enters the little shifter box on driver's side of the ZF, accessed by pulling the 2 rubber plugs from underneath that box. All are easy checks and possible sources of slop. But you may still have a slight misalignment of the linkage needing adjustment. I think when things heat up and things get in motion & under load, all that can change slightly.
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