Since it's rpm specific, that eliminates everything after the output shaft on the tranny (driveshaft, rear end, wheels & tires). Since it does it in neutral, then that eliminates the transmission after the input shaft. That leaves the engine, motor mounts, flywheel, and clutch. Check the motor mounts first. Since your engine relies on external balancing, I would suspect either a problem with the harmonic balancer, or the flywheel. Check you timing, and make sure the marks haven't moved. Then rev the engine to the point of vibration, and observe the balancer. If it looks fine, then I'd go after the flywheel.
Take care,
~Chris
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