This is a tough one to explain on a keyboard. From out of the back the trans , the driveshaft angle should be "down". Meaning the driveshaft goes lower than the yoke. At the diff , the angle should be "up". Meaning the drive shaft goes above the pinion yoke , but the yoke and driveshaft are not in a "straight line". To work properly , ( as I'm confident you know from all of your noted effort to fix this) U joints need "angularity" to function properly. A common problem is when the pinion angle is "higher" than the driveshaft angle which produces two "positive" angles instead of a positive and an offsetting "negative angle". "I think" that is what you are explaining IS present , yet you still have the vibration. If that is true , you may want to look at the axles themselves. I had one with a VERY slight bend that gave me fits in my drag race car at 120 mph trap speed.
'66 GT350H-3 time cover car - Car Craft July'77,Modified Mustangs and Fords Feb2011 ( w/article), Mustang Monthly June 2014. Bracket raced by me for the last 43 years. Yes it is a real one.
'68.5 R code GT fastback
'67 Ranchero 408W -427 glass hood
'78 F350 "Oleynik" car hauler
Ford Experimental parts collector