Rear shock for spesh xc comp

Im gonna buy a new rear shock for my XC COMP, probably a fox RP2 or RP32 but i was wondering, do they have to be custom tunned or can you just buy one straight off the shelf.
Reason i'm askig is because i spotted this about the shock i have now -
Rear Shock Brand:X-Fusion
Rear Shock Model:02RLA, Custom tuned for Specialized, rebound adj. w/lock-out, 7.5x1.75", 2 bearing upper shock mount

I know you need the correct length etc but the above sounds expensive if you have to go through all that custom guff

Any help would be great
and keep an eye on ebay for second hand ones too! RP23 is a deadly shock but it cost a fair bit!