Driveline vibration?

When I'm working on a vibration like that, I'll jack the truck up,put it on stands and try it.I'll have a friend run it at the speed it vibrates and I'll watch it.Remove tires and drums if need be,what ever it takes to narrow it down.Even rotate the drive shaft 180 and try it on the stands.All this eliminates road interference and tires.Hope this helps.