Try clocking your driveshaft 180 degrees from it's current position relative to the pinion. Modern OEM's balance the driveline as a whole, trans to differential. That's not really done on these older cars and there's a lot of reciprocal weight in the ring and pinion. Sometimes you need to move an otherwise balanced driveshaft away from the resonant phase of the differential.
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-Jason
1969 Pontiac Firebird
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