Every time I go in mud, or water my rear differential fills up with water, and has a black sludge in the bottom when I drain it. its getting tobe a real pain in the @$$. is there something I can do, other than not go in mud? The Highlifter guys are putting trailer bearing grease in their rear diffs, but they also say to not go faster than 25-30 mph. I do an equal amount of trail riding and mud riding. Im contemplating whether or not I should grease my rear end!