I'm not familiar with SQL Server, so I cannot tell whether it has some special capability to synchronize with AD.
What I know is that there are a few different solutions if you want to track changes in AD using LDAP. You can find an overview of the available techniques in this MSDN article.
I've recently open-sourced ADSync4J, a small Java library that can help you implementing the third technique mentioned in that article (Polling for Changes Using USNChanged). However it won't be of much help if your target platform is not the JVM.