One problem related to differential unit has been reported for the 2004 Toyota Sienna. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2004 Toyota Sienna based on all problems reported for the 2004 Sienna.
Car rolled back while on a slope with parking brake fully applied, in park, and engine off. Rolled back about 6-9 inches as soon as footbrake was released, with a child immediately behind car at that time. I later found the problem is a worn rear differential front support arm used on all Sienna awd's, and likely other Toyota's that share the same awd drivetrain design. Per the Toyota owner boards, it apparently commonly fails and sells for over $200 for just the part. However, it really concerns me here because when it fails, it allows play in the driveline, which then results in the car being able to roll for 6 inches plus even w/ parking brake applied until it reaches the limit of its play. On a slope and with someone just in front or behind the car that could result in serious injury or even death by either impact or crushing. Dangerous design and poor reliability.