I did that exact thing, I cheated a little when I bought the parts at the junkyard... I bought a complete axle for ~$100, took the disc brake components (brand new pads) off of that and put them on my Ranger (running an explorer 8.8) then sold the new axle converted back to drums for $160, so including buying new parking brake shoes I got paid $40 to convert to discs...
There are a couple notes on converting, stock the brake line goes from the drivers side tire up to the frame rail with a soft line, if you are building an offroader that won't allow for much articulation, so grab 2 passenger side soft lines while at the junkyard (or get another from the parts store) and a couple flare nuts and convert to the stock Ranger or drum explorer center drop brake line, pretty simple. The other note is the axle shafts seem to be a hair longer on the disc brake axles, for the first couple thousand miles after I swapped using my stock drum brake axle shafts I had a screeching noise from the rotors scruffing on the outside of the parking brake shoes. It's more annoying than an issue so it's nothing to worry about...