I know that this might sound a little odd. But I had a 86 ford van that did the very same thing under the same conditions. If I put the tranny in neutral and coasted, there was no vibration. Under load it would about shake you out of the seat. Replaced the U-joints, carrier bearing, Balancer, belts and pullies. Still not any better. Then after driving it for a couple of weeks I needed to put new rear shoes on it and the axle bearings were wore out! I put new seals and axle bearings (and new RR axle) in the rearend and was amazed that it was the original problem. When it was under load the axle was twisting in the housing and trying to ride up the housing causing the vibration.