Is your Box a stick. Have you seen any oil from around the trans where the axle connects. The diff has about one liter of 90 weight oil. If the diff gets low you can develop a roar like noise from it and only notice it at a higher speed. I would jack it up, look and see if the boots are not broken and check the trans. By narrowing down the problem areas you closer to solving your problem.
PS: I just checked on how to change the rear wheel bearing and that is a lot of work. You need to press out the old bearing and press in a new one. Find a Indy near you.
Check here to see how it's done.
http://boxcar-racing.com/forum/index.php?topic=562.0