The only problem with an older diff is finding one.
Everything else about it is still the same Toyota tough.
They will take all G-series diff centres and LSD's.
I have a 90's Hilux Surf 4.300 LSD in my RA25 axle.
Yes, you can still convert to rear disk brakes and 4 stud and
keep it legal. Depends where you live but getting an engineer
involved when you start the brake mods is a good idea.
Mitch.