vibration at 25-40 mph

The vibration is only in gear, so it's not anything in the engine. Just now I ran the car on stands with the wheels and drums removed (wood block under brake pedal) and the vibration is the same. So I know it's not the rims or tires. Also, there's no vibration with the transmission in neutral, so I know it's not the engine or exhaust.

I did replace the pinion seal before I installed the differential. I reset the bearing preload so that the turning torque was the same as before (about 6-8 in-lbs), and there was no play in the pinion.

Here's a slo-mo video of the pinion turning while the speedometer reads about 30 mph. The collar wobbles, but I don't know if that means anything.

[ame="https://youtu.be/fmb-uK1IEMc"]https://youtu.be/fmb-uK1IEMc[/ame]