Following "Schedule B" in the Jeep owner manual (heavy driving), I change my differential fluids every 12,000 miles. but should probably been done after crossing deep water... though I have not always done so. There are seals to keep the water from getting in.. so I don't think it is really necessary to do so every single time you do a water crossing... that could get expensive. But if you been in the water quite a bit or everytime you go wheeling.. you might want to change it before 12,000 ...