The stanchion isn’t a sealing surface, how bad is it? It sounds like there’s some grit or similar stuck in the collar/topcap area. At the very least it can’t hurt to pop that out and have a look if you’re going to write it off otherwise.
You’ll need to follow the beginning stages of the SRAM instructions (they’re on youtube in four parts) until you get the post lower and the bit where you put the air in off. Then carefully remove the lower bushing (at the base of the stanchion) and slide the top collar off. You can the give it a clean inside and inspect the stanchion properly. If it’s really gone, just replace it. If it’s scoring, you may be fine with just a fresh grease or maybe a new top collar.
All you’ll need (I think, from memory) is:
– circlip pliers (‘make ring smaller’ type)
– big enough adjustable spanner to fit over the collar
– vice and aluminium soft jaws
– some appropriate grease to reassemble. Rockshox use slick honey, I’ve been using the next closest thing I could find, can’t remember what.