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.
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Almost any air shock 6.5" X 1.5" (165 X 38mm) will work, any other size won't.
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