Changing the differential fluid isn’t hard, it’s just can be very messy! Whenever you go wheeling and you go through a lot of water. You should change the diff fluids to protect the gears and bearings.
Get you a LubeLocker differential gasket here. Highly recommend!: High-Quality Gasket here!
Pump to put oil in differential: Pump that fluid in!
Link to proper weight oil for the differential. You choose your brand!
Kobalt 3/8 Drive Ratchet set to remove/install the cover and fill plug. Check them out!