vibration problem

im chasing a similar demon in my 74 duster; 360 with 727 and 8.75. it vibes about 40-50, while in neutral or drive, the humming bee.. hmmm---hmmmm---hmmm.

did u-joints and im checking all the basics (wheels and tires, ect...) like fury said, its a process of elimination just like all trouble shooting. Im in the middle of it myself as are many others it seems.

It sounds to me however, that you are narrowing it down pretty well. you changed out the rear end, balanced the d-shaft with new joints...it appears that it is most likely driveline at this point, quite likely that tail shaft bearing took a good beating.

according to your second post... if it vibes in neutral while rolling down the road, logic would state it should be from the tranny back...you swapped out just about everything from the rear end to the tranny... so i'd again point to that tail shaft bearing...
...BUT...
I didnt see a post saying you tried tossing a different set of wheels on there yet...although unlikely, a bad tire can cause that. I chased down a vibration in my mothers beemer for months; it had a classic bad wheel bearing feel, it ended up being a crap tire. put new ones on and it was gone forever.