Could be a bad seal on the output shaft, or could be a bad bearing or could be that the bearing fit in the transmission end plate is worn.. The transmission probably can be rebuilt for a lot less than a new one would cost. If you can remove it, the work or parts for those fixes are not very expensive.. A good machine shop can cure those ills pretty fast. Someone has to diagnose the real problem.. but to fix any of those, the transmission should best be out of the boat. Good luck on the diagnosis.