1969 Dart 360, 727, 8 3/4. Noticed bad vibration after gear swap. Doesn't do it in neutral or any other gear besides the top end of 3rd. Suppose it could have done it with the 3.23 but its not often that I drive 130+mph. hard to tell in the video but its a viscous vibration/shake thats felt in the floor/steering wheel. It changes on decel which makes me think it has something to do with the driveshaft.
I had a similar problem
My rear u-joint wasn't perfectly centered in the third member yoke.
I carefully centered it...even measured it with calipers and the vibration went away.
