2004 specialized stumpjumper rp23 change

I have the 2004 specialized stumpjumper expert fsr and I have read the best change to the rear shock is a rp23. My question is i have seen a couple for sale, are they all the same if not which one do I need. Found one off a trek super fly for 50.00 and not sure if it will work. Trying to get all the info before I spend the money.