Where in the States did you rescue your dog from? Depending on the area, you should probably check not only for Lyme's but for the various ehrlichias and anaplasmosis--especially if your dog got into ticks or was in a place where ticks are common. There is a 4DX test put out by Idexx that tests for exposure to a common ehrlichia, Lyme's and anaplasmosis--it's a cheaper test that doing all the titers separately.
A CBC might hint at anaplasmosis--look for platelets and white blood cell counts lower than normal or even just on the low end of normal.
Good luck with your dog, buschgobbler--and thank you for rescuing her!