When I take off from a complete stop, I've noticed it more often while turning left, although it happens if I go straight, also. It starts out around the 2mph to 5mph range with a feeling as if something heavy is shifting in the trunk, starting from the leftside and then shifting backwards and then a "clunk" sound. As I begin to go faster, it happens at least once, sometimes twice more. It doesn't happen all the time but it seems to be getting more noticeable.

How are your rear upper control arms?

I would look at both front and rear control arm bushings, since sometimes these noises seem to come from places they aren't really coming from.