86 C-30 Dually Vibration Issue....

[dustykey]

Hopefully someone here can help me. This is wearing me and my wallet out!!!!
86 C-30 Dually Crew Cab Choo Choo Custom is vibrating and it starts phasing in at around 35-40 mph. That is when it is at its worse....but it never goes away, it just lightens up as speed increases. Truck was in rough shape when I got it. I have rebuilt the entire front end. By this I mean...tie rod ends, ball joints, steering arm, idler arm and steering gear as well as replaced a worn out rag joint. I have replaced front wheel bearings and races. Replaced all U-joints and the carrier bearing and had driveline balanced. Replaced all 6 tires with new BF Goodrich and had balanced. Transmission output shaft has no movement and I have checked the harmonic balancer and it seems fine. Will not vibrate when in neutral and engine revved up.....
Is a lost weight off of a rear drum a possibility? Bent axle?
Any help is greatly appreciated.....