High speed vibration and grinding noise

[whtgmc68]

I need some help with my 68 GMC LWB 4x4. I put in a Smedding Peformance 383, Dodge 27 spline with GM input shaft NV4500 and matching fixed yoke NP205. It had a mild vibration above 70 mph that progressively got worse until it become awful at anything above 45 mph and I found a failed u-joint at the rear transfer case output.

I replaced the failed joint and the truck was a pleasure to drive up to 70 mph so I took it out on the interstate to see what it was like above 70 and that is where everything seems to have gone south. The truck drove great until the speedo hit 80 and the vibration came back along with a serious whine (almost a grind) that seems like it is in the transfer case. Once I dropped below 80, the whine and grind went away and there is no vibration. Truck is smooth as glass as long as I stay under 80. The truck is lifted 6" and the engine was set back 2" during the swap so i dont think driveline angle is the issue. I have checked to make sure the hubs are not locked and the transfer case is in 2H.

Anyone have any ideas? The entire driveline only has about 2,000 miles on it.