One more area that sometimes affects shifting, and has nothing to do with the ZF. If the motormount rubbers are deteriorated, the engine can shift around relative to the shift-shaft support under the left header. This area NEVER gets lubed and can bind up when the powertrain moves.
Conversely, I've found Panteras in which the shift shaft support nut loosened up, causing the exact same problem as the support moved. Only takes a minute to check...
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