One problem related to tailgate has been reported for the 2008 Toyota Sienna. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2008 Toyota Sienna based on all problems reported for the 2008 Sienna.
In very cold weather (approximately 20 degrees or colder), the automatic tailgate on the back of the minivan malfunctions while parked. It will go up but as soon as it gets to the full open position, it comes back down and will not stop, even if you have your hand or body in the path of the tailgate door. This has happened to me numerous times over the last seven years (the last time being on the evening of 2/12/16 in bloomington IL) and the first time I barely was able to pull back before the tailgate would have crushed me. In summary, the tailgate door malfunctions in very cold weather (as it won't stay open) and it appears the sensor that detects something in the path of the tailgate door also is malfunctioning since it will not stop if something is in its path. I have submitted this to Toyota via occasional user panel email surveys they send, but I don't know if they are looking into it. This needs to be investigated before someone gets crushed, injured, or killed.