Why would you be screwed, a US 4.11 diff is somehow different from a JDM 4.11 diff
There are 4 common differential ratios, all of which can be easily found. The hardest is probably 4.44 which I just so happen to have for extra parts. 3.54 3.9 and 4.11 are fairly common so it wouldnt be so hard to find another. Even 3.7 is very easy to, but only prior generation subarus use them.