Storm said:
I won't beat the STX LSD allowances to death. Neither my 96 or 98 has an LSD, other years surely don't as well. The RAREITY comes with the years that DO have the factory LSD.
This applies to ALL the diffs not just the front or rear ones. All AWD Subarus automatically have an LSD in the center which is enough to disqualify them from touching any of their LSDs in STX. This modification to the STX Rules (it's in the April 2002 FasTrack) was made to allow the "AWD" cars that have 3 open diffs from the factory to add one LSD.
That said, the 98, 99 and 2002 2.5RS models should be able to perform an update/backdate to the 2000-2001 2.5RS rear LSD legally in STS/STX without a problem (since they are on the same line in DSP and it is in line with STS's "factory, viscous LSD" rule).
In fact, if I can find a good deal on a 2000/2001 rear diff with axles this season, I plan to do just that to my 1999 2.5RS in STS.