[IC27A] Alignment

[mwnmlr]

I have a M-18 Universal diesel and standard 2 bladed prop. Every year I have 
been getting a little more vibration and I am trying to track down the source. 
I had the usual things check such as cutlass bearing, prop balance, and engine 
mounts. I have heard that I should also check the prop shaft alignment. When I 
am sailing and have it in neutral the prop spins freely. Could it be out of 
alignment and prop shaft still spin easily?