One problem related to steering wheel vibrate has been reported for the 1998 Toyota Sienna. The most recently reported issues are listed below.
My wife was driving a 98 Toyota Sienna with kids on the freeway in a comuter lane. All of a sudden the steering wheel started vibrating and locked in left position. As a result they ended up crashing in the middle divider head on. The van took a 180 degree spin and hit from the back. We have done 99000 miles on this van in last 5 years. This is totally a malfunction or mechanical failure of steering wheel. We have done long trips with our van in the past 5 years and never had problem. Also the air bags did not open up even though the impact was so severe.