Are you sure the noise/vibration is coming from the ZF? If you are; since the issue occurs at a particular engine RPM, regardless of speed or in a certain gear, it probably has something to do with the input shaft. If you’re lucky, it’s external, which means pilot bushing or throw out bearing. Otherwise, perhaps an input shaft bearing. From there, it gets more ugly. Either way, you’ll be pulling the ZF to diagnose. Dumb question: is your R&P safety wired?
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