I had more water than oil in the rear diff. Any other way it got in there besides the vent tube? Also they need to learn how tight fasteners need to be. Broke a ratchet trying to get the filler plug out. Needed a pry bar the first oil filter change too.

it could get in via the axle shafts but oil would likely be coming out if that was the case

You can barely tighten those plugs and they will be a bear to break loose after a couple years.

I would check vent tube connections and routing.

[2015 Z Force 800 53/Trail EPS]

I have often wondered if it were possible for water to enter through the 4WD actuator housing some way. Besides the vent I know on Brute Forces the axle & pinion seal design will allow water to enter from the outside when the static pressure of the water outside exceeds the oil's on the inside. Remember, water is heavier then oil. That intrusion through the seals happens when the diff is under about 8" of water. Food for thought when you are in deep water....with any machine.

NMKawirider, you might be on to something. My vent is up high and haven't seen any oil leaking. The front was OK but changed it anyway.