Sounds as if the gears are grinding when in heavy

Sounds as if the gears are grinding when in heavy traffic. Rides well if you acelerate from 0, but if moving at 5mph and you acelerate to catch up with the car in front, it sounds as if the gears are getting stuck and grinding (like a meat grinder). The car has 31K miles so we don't use it much, but it was used to launch a boat a few times. It was hosed off every time, but could it be that the salt got into the gears?