welcome to the woderful world of owning an ae86. u could use a diff out of a 82-85 celica, but it has drum brakes and a taller final drive. but it will bolt in to get you buy.

you could also use a diff out of an sr5 corolla, but its weaker and has drum brakes.

if you want the same thing that u have right now then its corolla gt-s only.