When swapping rear differentials, the question is often posed: "What axles should I use?"
A lot of controversy and confusion is brought about by this question and I think I may have a definitive answer on the subject.
First of all, you can reference my thread here
to find those cars that had a rear LSD.
If you are equipping your car with an LSD, then chances are, you are going to need a new set of rear axles. Your current axles WILL bolt into the new differential, but they are not the same. You can see the spline count in this thread here
to verify that they will appear to fit.
To select the axles needed, my first suggestion would be to source them when you purchase an LSD from any chassis except the GD chassis (unless, of course, you are converting your GD chassis to a rear LSD). If you already have the LSD and are in need of the axles, then refer to the first thread indicating which cars came with an LSD. You must note which type of axles are used (female or male) and match accordingly. The Legacy, Forester, pre-2002 RS and the 2002+ WRX Wagon should all share the same rear LSD axle dimensions and be interchangable so just find a car on the list and go for it. Some junkyards are VERY ingorant as to what works and what doesn't so to be safe, demand the exact car and don't settle for substitutions. It took an unnamed yard two different tries before I finally cancelled and went with somebody else.
Now for the next question: "Why do I need new axles if the ones I already own will fit?"
The answer is their lengths. The rear LSD axles are 1cm (10mm) shorter than the corresponding open diff axles. While your open diff axles may bolt in and appear to fit, it could induce binding or other unforseen problems. People have reported using open diff axles with LSDs and vice-versa but it should not be taken as rule.
Lastly, "Are there any axles that are better than another?"
The truth is, the CV strength should be equal between all of the Subaru axles. While there are visible differences, who has heard of a Subaru mulching a CV joint? Diameters will also vary with the WRX axles being the thickest, then the RS/Legacy/Forester LSD axles, then the remaining open diff axles. Again, who has heard of a Subaru twisting an axle? Probably not a significant concern.
To quiet the argument of the open diff axles fitting, here is a picture of some 2001 2.5RS rear LSD axles compared to rear open diff axles from two different cars. Note the length differences between left and right and between LSD and open diff.
One final note: notice that while the left-hand RS axle is shorter than the right-hand RS axle, the actual axle shaft is longer than the right-hand axle. The axle shaft lengths are given in the newer FSMs (2002+) but have no correlation to the actual axle length.