42RH/A500 questions - 4WD vs 2WD

The overdrive assembly needs to be swapped to do this, but you have to find a rear wheel drive trans to get those parts anyways, so just wait for a whole one to show up.
Check with your local full service auto recycler or junk yard. They have a parts locator where they can get one shipped in from another yard that has one. See if that cost is acceptable.