Well, I fabbed myself up a couple of locked diffs a couple days ago, and I thought I'd share how I did it...it was pretty simple
first off....the whole I'm not responsible for damages to you or your xmod if you do this. when cutting/drilling shards of plastic and metal are flying, if you don't wear safety goggles and loose your sight, it's your own problem...etc. etc. etc.
1. remove diff, however you'd like to do this, it doesn't matter, just pull it out.
here's the diff, dremel cut-off wheel, and spindle. respectively:
2. cut spindle in half, I used a leatherman and dremel cut-off wheel, worked quite smoothly, but however you'd like to do it is fine
3. Drill a 1/16's inch diameter hole through the diff. making sure to drill it between 2 of the 3 spider gears, so as to not destroy one of these.
4. insert spindle through hole
5. replace bearings (or bushings) and reinstall diff
now, the spindle can be removed to unlock the diff, however (at least with both of mine) some debris will stay in the diff, and cause it to bind some. I'm not sure if this will cause damage, but it will effect future functionality
also, this would probably work for the gen 1 chassis, but I haven't tried it (and don't plan to, my skyline likes it's open diffs)