This is getting frustrating. Found some issues on the rear prop shaft - both U-Joints were missing their zerk fittings:
So I decided to swap them out while I was tightening up the CV flange bolt. Dropped the shaft, removed the transfer case side first. U joint was OK, but I had a new one to replace anyway.
Went to remove the final drive side u joint and noticed one of the caps was totally different than the other 3 on the u joint:
(this was quite rusty and that's what caught my eye)
vs.
(this had a retaining clip, I had already removed it.)
One of these things is not like the others....
My guess is some lazy car owner or mechanic didn't want to mess with that cap and left it in. Anyway, replaced them all with new u joints and bolted the rear prop shaft back up and went for a test drive. Vibration is still there. Begins at 35mph, and holds steady up to about 70. It's not horrible, I can feel it in the floor and there is a background hum.
the strange parts are it totally goes away when you turn right. You really have to move the weight of the car to the left side, but it goes dead quiet. I'm beginning to think it's just going have to be that way, I can't see anything else that might be worn that could be causing it.
If anyone has any thoughts, i would love to hear them.