I'm picturing (in my optimistic way) obtaining a complete FJ rear differential and spending a weekend swapping it into my T4R. Of course, it's the details that would kill me, like brake line and wiring bracket issues, if they exist. I suppose a good first step would be to crawl under an FJ and document all the essentials such as flange-to-flange dimensions, bracket locations, etc. and compare them to my T4R.
An ARB locker starts looking pretty good...