Hmmm. Removing the NVH kit usually stops the vibration so what next?
My machine is fitted with a chain final drive and if the chain is slack it will snake when running slow and no load - can this happen with the belt drive? It might be worth experimenting with belt tension.
It may be worth checking the flange bolts on the hardy spicer joints in the prop shaft but I can't see that problem going away on its own.
Did you dip and re-engage the clutch while it was happening? If so did it make a difference? A friction plate breaking up perhaps?
I am clutching a straws for an answer here so I think you may have to do the old trick of letting it develop - but carefully!
Again - good luck.
