Currently running a 3.91 gear in a 8.25 rear behind a A-833 OD trans (1.00 ration 3rd/.67 ratio 4th) and mild 318 in my Duster. Driveshaft is 2.75” diameter. After the gear change last month, above 2300 rpm/65mph in 4th/OD, there was a minor vibration/resonance. Had new u joints put in and re-balanced. According to ship, the driveshaft is not perfectly true, but almost with very little runout. That improved the issue down to what I can describe as more of a medium frequency resonance that actually intensifies slightly as I let off of the gas, and not really a vibration. It’s is still there to a lesser extent when I pop it out of gear. Feel it more up in the front under my seat. Rear tires are new and balanced, same for front.

Motor mounts are about 8 months old and ever since, the engine has always seemed rougher when free revving in neutral with those mounts, even before the 3.91 gear and prior to that, the 4 speed swap.

Does this sound like pinion angle slight Imperfection, or driveshaft half critical speed situation, or maybe slightly worn axle bearing? I am still driving the car, because I don’t run it on the dragstrip and it isn’t shaking me to death, but I don’t want to risk the driveshaft exploding (if that is even a legitimate risk or fear).