Definitely replace your Rear lower control arm. In a collision like that, it is designed to fold and to save other parts of the car. Chances are this is the only part that is damaged.
I would also check your rear upper control arm, they're pretty weak as well. Check your subframe where the two control arms connect to for any damage. Check the rear trailing arm where it connects to the chasis. Check your halfshaft. Check the rear shock mount, too.
Basically, get your head under there and check out everything. It's tough to tell what could be damaged.
Edit: yah... what bitcore said ^^