An easy fix to diff oils freezing due to water contamination is to add a friction modifier, some contain antifreeze agents of some kind. I’m NOT in any way shape or form advising anybody to add antifreeze to oils.Happened on my fourtrax 400AT and still is its always in the winter. I think its the final drive/rear diff. Tap it lightly with a mallet or hammer w/piece of wood between to try and loosen anything that could freeze inside. After I ride for a bit it thaws out from the friction, however if you don't ride in water/haven't been in water then im not sure.