One problem related to differential unit has been reported for the 1998 Toyota Sienna. The most recently reported issues are listed below.
The contact received a warranty extension for transmission problems. The vehicle was making noise. The transmission had no other noticeable problems at this point. The mechanic, who looked at the vehicle, stated the transmission will have to be replaced soon. The mechanic said it was making noise due to defective differential and the transmission was slipping. The service manager stated that Toyota would provide the part, and the contact would have to pay for the labor. Update 1/4/2006 -.