HMMM -- Lots of possibilities
Could be the shift cabling & hardware , drive, engine coupler , or prop hub
You should be able to to o a few checks on the trailer or in shallow water
With the engine off -- and the shifter in drive -- the prop should spin by hand in 1 direction with a ratchet type noise // and not spin in the other direction // same thing for reverse but opposite directions
Does that happen ? -- If so the shift cables & HW are probably not the culprit .