Noise when shifting
 

I am getting a loud clunk noise when shifting from neutral to forward or reverse. Engine is idling at 700-750. In gear 600. Its louder when running on the trailer. I drained the drive oil today and noticed some black sludge on the magnets and a slight darker change in oil color. I am running Amsoil Marine gear lube. This lube is normally clear. My other drive was fine. Very clean oil.

Any suggestions
Thanks