Very hard to believe that: 1) an idle/stationary prop shaft could cause a vibration, 2) a prop shaft would cause vibration when decelerating, but would not cause vibration at other times.
How many miles?
Vibration at idle is likely engine running rough.
Any decel vibration from higher speeds?
Did decel vibration exist before pads/disks were replaced? I would check tie-rod ends for play and consider thrust arm bushings.