Well folks I have a problem.Although, thanks to you all you guys I do have the facts . From the information gathered I have a couple of choices for my 91 Legacy turbo that still doesn't have a working auto transmission!
I can put in a trans that has a 3.9 rear diff, the problem only legacy turbos and WRXs have that diff (as far as I know) both exspensive new and rare used.
Although, what if I put in the rear diff from a legacy L AWD I have found then the trans from it? Would it work and could I do it my self?
Also if anyone knows of or has a used trans from a Legacy turbo or WRX that is for sell, I'd like to know.
first of all your 91 should have a limited slip rear differential.
what is wrong with your transmission. if your differential is fine I would not replace it. 91's were rare in the way that they came with an lsd.
I will be selling my 94 5spd transmission with diff's, clutch, fly wheel. ss, and everything as soon as my 6 spd comes. it has 55k miles on it and has had neo synthetics ran through it. it is a really good tranny. just thought I would mention it. thanks, kelley.
The 92-93 n/a legacies with auto trannies have 3.90 gearing as well as all the turbo legacies.
Ciper is correct.....The WRX uses a 1.1:1 stepper for the rear final drive ratio.
What it accomplishes is, it gives the WRX more FWD power bias.
If you have a 3.900 final drive reduction in the front
The rear you have a 3.545 rear diff and a 1.1 stepper. That gives you a 3.8995 reduction in the rear. It is essentally a 3.90 reduction.....but the front has just a slight edge on it.....so the car will act more FWDish....
. From the information gathered I have a couple of choices for my 91 Legacy turbo that still doesn't have a working auto transmission! 
