Yep, done a lot. For the rear axle what works on a 2wd works on a 4wd.
84 is going to have a 7.5 axle and you've got a few options available. Mustang is probably going to be your easiest swap. Install a complete 2010 or 2011 rear axle that has disc brakes from the factory, but they are slightly wider 8.8" axles and will require at least a few other changes. The '96-'01 Explorer axle came with disc brakes, it's a little stronger but it's a fair bit wider and going to take a bunch of changes to install. There's information on all of it in the tech library as Eric said, do some studying and come back for clarification if needed.
You might want to just look at rebuilding and upgrading your drums. Your '84 may have come with 9" drums. 10 inch drums would be an easy upgrade and you can get finned drums with improved heat dissipation. While not as cool, it might actually be a better upgrade for you than Mustang disc brakes. The Mustang rear disc swap uses a small thin rotor that isn't near as good for hauling abuse as the larger vented rotors used on most trucks and SUVs. Heat is the enemy of braking, some heat is good, too much heat is bad and leads to fade and fatigue (aka even worse braking). The 10" drum, especially the finned version, might be better at dissipating heat than the standard rear Mustang brake swap.
I wouldn't just look at rear brakes. On a truck the brakes are biased such that the fronts provide most of the stopping power. The rears do provide additional when loaded due to the increased weight on them, but the system is still biased towards the front. It'll take a bit of work, but later model (95-97?) Rangers came with dual piston front calipers. Would probably be an even better upgrade than rear disc, especially if combined with 10" drums.
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