A J.D. is considered a terminal degree (there is no further degree beyond a J.D.; the S.J.D. is considered a different track), so technically you do have a doctorate.

But ethical rules are questionable as to whether you can call yourself a "doctor," because it's potentially misleading. So US lawyers are in the strange position of having doctorates, but being unable to say so.

Here's some more info: http://abajournal.com/magazine/lawyers_are_doctors_too/