Hello, first let me introduce myself. I recently had to sell my 1990 Ford Bronco (hence my name) because it failed emissions six times and I gave up. I bought a 2004 Explorer 4X4 with 93K for $7,200. I am very happy with my new truck. It is very clean and looks like new. I only noticed one thing - a slight whine from the rear end at 55 mph when I am on the gas. It doesn't really sound like a bearing. I read somewhere that Ford put a lighter weight gear oil in the diffs to increase fuel economy, and if you put in a heavier grade then the whine will go away. Is this true? If so, what should I use? The whine is barely noticeable and doesn't bother me, but if it could be eliminated easily by changing the oil I'd do it. I also read that someone did this and a week later the diff blew up. I'd rather have a very slight whine than a diff blowing up! What should I do here? Thank you!