These are two words that have baffled me for long. Dependency is given as 'excessive dependence' in Chambers, but I would love to know how the spoken usage is. My guess is dependency has a political touch to it. In addition, there is an independency as well in the dictionaries, though it does not sound popular.
I think "dependence" and "dependency" are like "competence" and "competency". Both are nouns. Yet, "dependence" emphasizes the quality of being dependent. So, it may also be abstract. Whereas, "dependency" focuses on the state of being dependent. It is likely to be concrete.