[Marshall V. Daut]

Check the pinion gear nuts. The repo "keeper" washer's ears are too short to bend over the two nuts' straight sides. Consequently, the nut closer to the transmission loosens up and begins to back off the threads of the pinion gear. Then the nut next to the rear end center housing will loosen up because the locking nut ahead of it has moved away. When that happens, the drive shaft starts to whip in the rear end, which causes vibrations UPON DECELERATION (because the torque is off the drive shaft) and scraping or knocking noise. If allowed to continue, there could be serious rear end problems and/or a broken pinion gear or bent drive shaft.
To check, jack the car up and place jack stands under the rear axle housings. Lie on your back right beneath where the torque tube mates with the center carrier housing and while someone hand spins the rear wheels, listen for scraping or knocking where the pinion gear is located. Have the assistant rock the tires forward and backward. If you hear scraping or any metallic knocking noise other than the bearings turning inside the rear end housing, the front nut on the pinion gear may have worked loose, followed by the rear lock nut.
This has happened to two Model A's I have fixed. I at first thought of a bad universal joint because the symptoms during deceleration are the same. But since you have a new U-joint, we can probably rule that out. My money is on loose pinion gear nuts from the way you describe the symptoms. This is especially suspicious because you have recently installed a Mitchell overdrive. If the lock washer isn't doing its job and keeping the two pinion nuts in place, this could be the source of the vibration while decelerating.
Marshall