Look on craigslist for the rear axle. A couple months ago I found one for a friend, after a few weeks searching we chose from about a half dozen cheap ones (less than $200) all over the country.
I'm not familiar with the front axle on your older (than mine) truck. It's definitely worth looking into adapting disc brakes to what you have. I didn't see it as an easy or good option on the 48 axle, with luck yours will be.
Grigg
