Tajik, often called Tajiki Persian, is regarded to be a variety of Persian among Tajik people living in Tajikistan and Uzbekistan. It is closed to Dari in Afghanistan, which is considered to ben another variety of Persian. Tajik, Dari, and Iranian Persian are all believed to be the continuation of the ancient language, Middle Persian.
Tajik is the official language of Tajikistan, as well as a national language of Tajik people living in Afghanistan, China, Russia and etc.
IU doesn't regularly offer Tajik courses, but you could visit the website of Central Euroasian Studies and get updated information about the learning of Tajik.
CeLT has a series of Tajik themed audio recordings as listed below. Due to the copyright restrictions it requires IU logins.