I have a 98 Ram 1500 2WD with an auto. tranny. A couple weeks ago I notice a humming noise that instensifies with speed. Its not tire noise so the only other thing I could think of is the rear differential. I changed the gear oil yesterday and the gears in the back look OK. Anything that I'm missing or is there some maintenance that I am missing? Thanks, Andy. By the way that was the first time I've changed the gear oil and the truck has 50K on it.