[Mike Taylor 3601]

Usaully driveshaft will vibrate at two points on a run,a certain rpm which it will vibrate when passes that rpm accelerating then at that rpm when decelerating.
I still think sounds like gear noise.
Dan,
If can mount dial indicator and check driveshaft for run out on each end, if it runs around .015''or less should be smooth.
Built driveshaft awhile back that couldn't be made smooth,had to build another one with bigger od tube and thinner wall thickness to fix it
Mike Taylor 3601