diffs can be rebuilt easily enough, my gtr rear diff has new bearings and seals, along with nismo clutch packs. My g box was also noisy but i just slapped in another gtr box, its a 32 box and took the chance, at 700 bucks they are cheap and give it a go.
My rear diff is fine still has a slight whine at around 100 km/h, but the problem is its a worn gear set if it was new be better but i dont want to buy new gears prob cost more then half the car haha. Also depends what backlash you put in it, running it a bit looser lets more oil in for long life, but too much is detrimental etc.