I tightened the nut and it did tighten the flange a bit. I took it for a test drive and it may be a little better but the vibration is certainly not gone.

The thing that perplexes me is that it does not happen if the clutch is depressed whether the truck is in neutral or not. The fact that the clutch has an effect has me wondering.

Oh, well, now that I'm convinced it not the u-joints I'll just drive it until it breaks.