As already said, replace them ASAP.
If the joints aren't knocking then you will probably get away with stripping them and cleaning them with petrol/white spirit.
You won't be able to get the shaft out of the hub without a puller on a mk3 gti/vr6, anyone that says you can tap it out, hasn't done the job.
+1 im doing it #atm and the gti/vr6 splined driveshaft bits are 'bonded' to the hub, according to elsawin. I welded up my own puller, just like the proper one+ thought i could get away with m10 bolt to push put the shaft, no way sheered right off, now im goining with M16, and it had better not sheer that off!
You do need 'special' splined/star bit to unbolt the inner cv from the gear box hub, i cut away the boot to get better access.
isnt too bad a job just right tools as usual, dismantle cvs and give them a good clean out, you should get a pack of grease to repack them with. refitting is reverse of removal lol