If there is no leaks, check and make sure your axle vent is clear and working...I just went through this exact thing...smelling burnt gear oil. I had no leaks, fresh gear oil, cleaned my axle vent. It turned out to be my right rear brake pads, over adjusted way too tight to the point of being almost completely fried. It happened over a couple of weeks time. Put new pads and hardware on both sides, no more smells whatsoever...seems like I'm checking my adjustment everyday now!