2001 Chevy S-10 hum/vibration at speeds 25 and up

I have a 2001 chevy s10 with 2wd. About a month ago I began to notice a hum and vibration coming from what seems to be the rear end of my truck. Before I came to this conclusion, I repacked the front wheel bearings with grease and changed the brakes. Now I'm determined that the noise it coming from the rear end. Two days ago I changed out the old gear oil in the diff and inspected the gears to make sure there was no play in anything. Nothing, everything looks ok. My u-joints are still factory sealed and have been there since the truck was manufactured in 2001 so maybe that's the problem, but I didn't notice any unusual play in any of the joints. While I drive I also notice that my passenger side mirror is vibrating and the drivers side is not making me thing that it could possibly be the passenger side axle bearing. Any ideas or suggestions would be greatly appreciated, this problem has been consuming all my time and it's driving me crazy!