hey mate welcome to the forums.
first off the auto diff is a 4.1, not all that great but better than the manual one being 3.9.
pretty much any diff that bolts into an ae86 will bolt into a ke70 (with the appropriate tailshaft).
the diff to get is the "T" series diffs. there is stacks of information about and there are alot of aftermarket centres available. though from what ive gathered they they arent that much "Stronger" than a ke diff.
T series diffs come in a range of different cars, but the ones to get are from a toyota T18 and from the japanese ae86. (ausi ae86 comes with S series diff, which is useless like the ke70 diff).
there is a "big" and "small" axel version of the ae86 diff, i dunno what is what but just be aware there are 2 sizes. one being the same as the t18 diff.
as for ratios, there are alot of aftermarket ratios for T series diffs. 4.3 being fairly common, 4.5 i think is around, and 4.7 also relatively common.
id suggest jumping on google and hitting up it up as there will be stacks of info out there!
Welcome again, and make a members ride! i wanna see a rally car!