Great video
I had this same isssue, it ended up being 2 problems.
My drive shaft was in balance but the driveshaft tube was out of round, even new tubes need to be corrected. I took it to 2 differant driveshaft shops before I found this out from the second shop.
The pinion yoke on my 8 3/4 was old and sloppy not allowing proper fit on the pinion gear splines. To the naked eye it appeared OK. After replacement of the yoke no more highway vibration.
