I have read that the Sienna AWD system uses an open differential for the central differential (and I presume the front and rear as well) and relies on four wheel traction control to prevent wheel spin. It is always engaged.
The 2004 Toyota Sienna has front wheel drive.
The 2007 Toyota Sienna has front wheel drive.
The 2003 Toyota Sienna has front wheel drive.
The 2011 Toyota Sienna has front wheel drive.
The 2014 Toyota Sienna has front wheel drive.
The 2010 Toyota Sienna has front wheel drive.
The 2005 Toyota Sienna has front wheel drive.
The 2008 Toyota Sienna has front wheel drive.
The 2009 Toyota Sienna has front wheel drive.
The 2001 Toyota Sienna has front wheel drive.
The 2002 Toyota Sienna has front wheel drive.
The 2006 Toyota Sienna has front wheel drive.