From my perspective, no sociology will not help you with anthropology, they have diverged a great deal in the past 50 years and approach the analysis of human societies with very different perspectives. I looked at University of Michigan's history/anth PhD, and it might be an ok fit for you - I would start contacting faculty members and maybe other graduate students there to get a better idea. You don't want to end up wasting the application fee if you are not a good fit for the department from the get-go.
My best advice echoes so47's - relax, and maybe think outside the box. Some of the best advice I have heard about the graduate school process is that at the end of the day, yes the PhD from a top program is amazing, however it's the gunner, not the gun. You can do great things elsewhere - graduate school is a huge commitment and it might behoove you more to find a department with faculty members that fit your research interests well rather than focusing on the program that is the most prestigious - especially for the social sciences.
As far as your letter writers are concerned, I would probably not advise to have them mention that you over-exerted yourself to explain your A-'s, as so47 said that shows immaturity, rather have them focus on what makes you the best possible candidate for PhD programs. Those letters should be singing your praises, not highlighting your pitfalls. However, unless there is some circumstance that forced you to overload your schedule (ie certain classes only offered once in a blue moon all happened to be offered this one term) maybe that is something they should mention, otherwise I would just want them to focus on the positives.
It is also beneficial to start contacting faculty members at the universities you wish to attend, figuring out who you would like to be your adviser, and getting your name and face known around those departments. Although certainly not a guarantee, I feel as though personal connection to the department ahead of time means your application is more likely to get a second or third look.