So, this just happened to my car. Scary as I felt a small shimmy on the freeway the night before, then the next morning my backend kicked over a foot when going over some railroad tracks. I got to my destination, and it completely failed when I was slowly backing up (seems this is the same for many). So glad this did not happen at freeway speeds!
My P85 was delivered in Feb 2013 and has a VIN in the low 4000s.
My service center said:
1. SB does not address my VIN (says vehicles approximately June 2013 to Feb 2014)
2. Right side link is also corroded and needs to be replaced.
3. Don't see any signs of stress or damage to left side.
4. I've never seen a rear control arm crack before
I replied that it was clearly the same issue, the SB said both need to be replaced, the problem is corrosion - not outright damage, and this is all over the forums and the NHTSA site. I got no reply for 24 hours until my third follow-up. I finally got a quote to fix the one side for about $2k (he said he emailed it the day before, but did not), and I approved it in order to get my car back. I'll have to decide if I want to proactively replace the other side later.
I will submit a NHTSA complaint after repairs are complete.
Overall, like others, I am frustrated that Tesla does not take more ownership of these obvious defects.