Hey Zach -
Based on your description, I'm guessing that your data source is a text file, Excel workbook, or Access database.
We access all three of these sources via the Jet (MS Access) database engine, and since it is a bit long in the tooth, it doesn't support the ability to do a distinct count. Fortunately, the Tableau fast data engine does.
You'll find that distinct count is there for you via Oracle, SQL Server, etc, too.