it's not the lower control arm; when those go bad, the ball joints squeak. it without a doubt the infamous track control (a.k.a. trailing) arm. this is a very common weak link on the box; the bearing in the end goes bad and the arm clunks and knocks around when you hit rough pavement. the cheapest place to get new ones is here (pap-parts):
http://www.pap-parts.com/prodinfo.asp?number=996%20341%20043%2003
alternatively, che (delubozparts) makes aftermarket adjustable replacements for around $275 per PAIR. i believe CJ_Boxster has a set. also, don't get an alignment before replacing these, as removal / replacement of this part will alter your toe slightly, requiring alignment.