It's actually relatively easy to replace the diff. Once you have the trans out you remove the two bolts that hold the gear sets in place on the case, and the bolt with the ball and spring behind it then remove the end cap and two lock nuts on the gear shafts, once you pull the 5th gear sprockets off you can unbolt the case and separate it. Once the case is off you can pull the gear shafts up a bit and rotate/slide the diff out. Not much to it really. Be sure to install new bearings on the new diff and clean up the magnet inside the case. If the trans is original you may want to consider a rebuild or replacing it with a lower mileage unit. You can find a JY trans for anywhere from $100-200 complete. Rebuild kits start around $800.