One problem related to transmission not engage has been reported for the 1998 Toyota Sienna. The most recently reported issues are listed below.
At interstate speed I had a few second drag and then complete transmission disengagement. Managed to get it off the interstate by letting it set a few minutes and being able to drive a 1/4 mile or so at a time. I have always taken very good care of the van including fluid changes. It only has 77,000 miles but Toyota refuses to do anything about it even though they are aware of the defect( because there warranty extension is only for 8 years 100,000 and I am over the 8 years.