vibration problem

" have a 76 Duster I bought 10 years ago and it had a 225/904 combo with a junk rearend. I built a 440 and put a 727 tranny in it and had Moser Engineering fix up a 8.75 rearend for it."

When Moser did your rear,did they build the whole rear including the spring perches? Could be they were installed wrong. You can buy wedge shaped shims from Summit and put them under the perches. Thick part to the rear points the pinion down.If it gets worse, try thick part to the front. These vibration issues are a process of elimination.