Is it possible you have a rear tire that is out of balance. With what you have done already you should be good to go.Driveshaft was rebuilt and rebalanced. The balancer on the transmission end of the shaft was removed when they balanced it.
Pinion preload was right at 25 inch pounds, which is what it's supposed to be for a new bearing (according to the internet)
Out Cadillac has a slight vibration around 65 through 80 and it settles out.
Originally it was a tremor at those speeds. Turns out CKW went pot hole plowing and bent a wheel. I sourced a direct replacement and it was real good until I noticed this. I am betting she whacked another wheel.