SQL Server log file continues to grow?
No. A differential never truncates the log. Please consider your recovery needs and choose a recovery model that fits those requirements.


  There’s still a widely held misconception that when properly in the
  FULL or BULK_LOGGED recovery models that full or differential backups
  can truncate the log. No. It NEVER happens.


http://www.sqlskills.com/blogs/paul/misconceptions-around-the-log-and-log-backups-how-to-convince-yourself/

I'd recommend easing up on the Differentials and start with scheduling t-log backups ever 15 minutes.

More information:

Once you get your t-logs backed up appropriately then you'll want to leave the shrinking alone. Whatever it grows to (usually within that 15 minutes) is usually the growth profile of what it actually needs. To shrink the log file only to let it grow again will cause unnecessary overhead and may impact performance. 

If you want to consolidate your VLFs then you need to find an outage window when you can shrink the log file and then grow it back to it's preferred size using a standard increment (use growth increments of 8000MB)