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Re: Driveline vibration
Hey skytop,
What you are hearing is called a beat frequency. Two vibrations are occuring at nearly the same frequency. They periodically amplify or cancel each other. I haven't worked on two piece drive shafts but the alignment could be the problem.
One thing to try. Put the rear axle on jack stands, level the front. Then see if the vibration occurs. It probably will. Then while it is vibrating, just touch the snout of the differential with a floor jack. If that stops it, the drive shaft is exciting a torsional vibration of the diff which is exciting the truck bed and thus everything else. That would be drive shaft alignment probably, assuming that the shaft is balanced correctly as you said.
It makes sense that a gear change would change the speed at which this occurs since it would require a certain drive shaft speed.
Let us know if you run the experiment how it turns out and we can go from there.
I ran into a similar problem with a friends Morris Minor custom built rod. We had to make a tuned passive damper attached to the axle to cure it. Yours being stock should be able to be fixed without that.
TR
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