is it a different carrier for 3.7 and 3.9 in the rear diffs?

nope, they are the same, but the inside of the 3.7 is grinded down a bit to fit in the limited slip.

grind in the housing or the carrier or? also i dont have limited slip

the housings and carriers are all the same (except for the slight grinding in the bottom of the housings that came with the LSD rears.

 

Turbone is referring to the ever popular LSD swap, that pulls the LSD carrier out of it's 3.70 diff, and puts it into a 3.90 housing with 3.90 pinion and ring gear. the grinding is needed for the installation of the LSD carrier to fit through the rear of the housing.

 

all ea81, ea82, late ea71, and the 91-93 Turbo Legacy rear diffs are interchangeable (provided you match the gears in your trans )

I have an 89 hatch. How do you know if it's a 3.70 or 3.90 diff? Can you tell by spinning the rear wheel(s) and counting the driveshaft rotations? Is there a specific model that had the LSD or is it hit or miss in the junkyard?

The gear ratio wil be on the cover. If it is an lsd it will also say lsd.

I have an 89 hatch. How do you know if it's a 3.70 or 3.90 diff? Can you tell by spinning the rear wheel(s) and counting the driveshaft rotations? Is there a specific model that had the LSD or is it hit or miss in the junkyard?

 

Turbo's came with the LSD as an option, it'll say LSD on the rear cover, if both wheels spin the same way its an LSD.

 

In your 89 hatch, if its a stick you have the 3.90 if'n its an auto its a 3.70