Originally Posted by Patrick Olsen
As discussed elsewhere (I think you were party to that thread, and now I don't remember where it was...), the '02+ WRX has a viscous rear LSD, and it was optional (I think?) on the '99-01 2.5RSs.
A VLSD was standard on the '00-'01 2.5RSs. I found this thread
by stimpy about swapping differentials and a referenced thread
also by stimpy about which models had LSDs. Apparently the '99 2.5RS had the VLSD as an option.