One problem related to differential unit has been reported for the 2004 Toyota Matrix. The most recently reported issues are listed below.
I just found out that the rear suspension cross-member (part # 51206-12190) in my 2004 Toyota Matrix is about to fail due to rust. While doing something else my mechanic happened to notice the issue. He showed me the problem and said it would be dangerous to drive the car. The car has low mileage for being 16 years old (about 8k a year). This car was built in canada and sold in new york state so it should have been designed to take many years of wet/winter road conditions. If my car had 16k miles per year it would only be 8 years old. Should a part that holds the differential up (not let it drop to the road) go bad in even 20 years on the road? I only found out about this problem by chance. Otherwise I could have been driving down the road and possibly had a fatal accident due to the rear parts of the drive train dropping to the road. The car looks fine otherwise. So how many other people who own the same car that is the same age, or newer or a bit older, have no idea that this problem exists? if even one owner of this car has an accident caused by this problem and is hurt or killed that would be very bad! and that person could have been me - because there has been no recall issued intended to check for this potential failure. I have spoken with Toyota about this issue and they told me I am on my own, that they couldn't/wouldn't help me. The two people I talked with never said that they would do anything to be sure other Toyota Matrix owners were informed about this possible rust issue.