You typically don't have to replace the two rear sensors unless you want to spend extra money. But I could be wrong, that just seemed to be the consensus out there. Is that code specific for the rears as well?
I got mine from my local German parts supplier but basically paid retail of $175 each, same as Autopartsway for OEM replacements. Meh.
I'm sure there are other cheaper DIY options to do them. Installation was about 40 min including jacking the car up.