I went on a search for differential parts this summer using phone numbers, here is my general experience:
There are a TON of MD(medium duty) truck salvage yards around the country and if you're willing to spend the money to ship, YES the parts for the rear end are typically available. Not sure what kind of rear end you have, but I'm sure you'd find something.
Brake cylinders are available. Go onto raybestos' website
http://www.raybestosbrakes.com/magno...=Medium%20Duty
Search for your truck and then search for those part numbers on rockauto.com. If you're really crazy, find cross reference part numbers and you might be able to find them for super cheap.
My personal opinion on the whole "modifying old MD and HD trucks for coolness" is basically get ready to spend a lot of money for a truck that might cost a lot to register, and still be outdated and slow.
If you need a tow vehicle, go get one. If you need a money pit with the style of the 50's....you got what you're lookin for.
P.S. I have spent the last 4 weeks working on the entire brake system on my 1967 C50 and let me tell you this 1) the costs add up, 2) the f&*$#@ng parts are heavier than hell and very tough to work with, 3) by this point in the trucks life, E-V-E-R-Y-T-H-I-N-G (at least for the brakes) needs to be replaced.
....and I'm only half done, not even finished yet.
Still a cool old truck and a hard worker, have fun.