I am responding to skylilymd's PM here so others may follow the discussion.
Skylilymd, yes, I am the one who posted the thread. It's at E450 Driveshaft Problem Fixed On The Road
It may help you figure out your problem.
I included photos in my thread so you can see what to look for. You may want to try my temporary fix with hose clamps to confirm your problem. If it reduces your vibration, you could just leave it at that. I've known it to be a permanent fix.
When the weight came off, we were going 65 mph on a smooth interstate. We immediately felt a vibration that became greater as I slowed down through 45 - 50 mph, then reduced as we slowed more. There was a loud hum with the vibration. Even after having it rebalanced, there is still a little hum that seems to be greater around 50 mph. It may be as good as it was when new and I just notice it more since I had to do so much work on it. When we lost the weight, it wasn't just a little hum. It vibrated the rear view mirrors so badly I couldn't see behind us.
Read the post and get back with me if you have more questions.