its deffinantly an idea... but reverse engineering to a problem that (many can argue) doesnt exist. personally, i think it could work. with the access to the tools, it can be done quite easily. i think if you started with a chromo shaft itd be quite strong. i find the weak part of D30 shafts, especially stockers, and ESPECIALLY on the outer, tends to be the ears.
but hypothetically... unless the donor parts came off of a good, running and driving WJ, id want new calipers/pads/rotors, minimum. if your XJ is like 95% of them, your buying new unit bearings regardless... it isnt equipped with the correct ones. WJ bearings, '99.5+ bearings, your getting new ones. or rather, i would. now... the cost of machining stub shafts probably isnt any more than having someone weld on the spacers. but every reasonable jeeper has at least one friend with a welder willing to do it.
but wait, you can just have 2 turned, best idea is probably to have at least one, probably 2 spares made. now what happens on the trail? your spares are useless to a buddy in need, or worse? you forget your custom parts and are very SOL... and if you ever need more or want to upgrade your axle shafts, your looking at more work.
as cool as custom parts are (ive machined my own custom knuckles and spindles), im a believer that one should try to stay away from custom "consumable" parts... such as axle shafts... especially D30 axle shafts...