Most people that want 4wd disk brakes on their older truck have one option - install a later axle. The reason is that closed knuckle axles w/o ball joints were never made with disk brakes - so no one wanted to try any kind of swap (too much fabrication). The only way I could see you getting disk brakes on your existing axle would be to have a custom bracket made up that would mount up later calipers. This bracket would replace the brake drum AND the brake drum backing plate. It is also likely that some machine work would be necessary in order to fit a later brake rotor, wheel bearings and locking hubs. This is something that Baer or another specialty braking firm may be able to help you with.
There are also some companies out there on ebay and elsewhere that specialize in REAR disk brake brackets for 4wd trucks, it is possible that you might be able to interest them in producing a bracket to allow you to mount up a disk brake.
I personally chose to go with new axles front and rear on my 66 3/4 ton 4wd, because there was a lot less work involved - and the old axles were shot anyway.
