Sounds like you're going to need to do a complete rebuild. The shaft is one peice with the box, you're going to have to remove the whole thing to get at it. It's much easier if you take the LF fender off first. In fact, the manual tells you to do it that way. There's really nothing too special about the innards of this particular box, for the most part the bearings go in like a standard wheel bearing. There are some special tools needed to disassemble the box and to do the final adjustments, but they're not too expensive or hard to get.
Here's a link to the manual section that might help you overhaul the box.
http://chevy.tocmp.com/1955truck2shop/55ctsm0901.htm
