If this is a new sound, its probably not a good sign. There are lots of Internet sites dedicated to how to work on a Ford 9", such as
http://73-79.com/tech/ford9/. These help you see what's going on inside.
Getting the axles out for inspection and bearing/seal replacement is no big deal, though actually replacing bearings will require a shop press and the right tools. Its a common job so there should be plenty of shops around you that can handle it if you choose to remove and reinstall the axles yourself after servicing. If the problem is in the 3rd member itself, that gets a tad bit more involved but is still something you should be able to at least remove yourself (though its bulky and heavy.) Any good 4-wheel drive or rod shop should be able to check out and repair a Ford 9". If it were mine, I'd change the gear
oil first and see what that looks like. If your diff
oil is real dirty, just changing
oil could help because the posi-trac needs clean gear oil that has the limited slip additive in it, or you need to add the additive. I would then run it by a shop that deals in these axles and have them give a listen. They may tell you its nothing more than a noisy traction-lok. If that doesn't yield success then I'd the pull the axles and inspect the bearings. And if all that's good, its time to drop the 3rd member.
FYI, if you wind up needing serious help, the folks at
John's 9 Inch Factory are awesome and build a great rear end, I've got one in my Hurricane and its friggin' bullet proof.