One problem related to differential unit has been reported for the 1998 Toyota RAV4. The most recently reported issues are listed below.
I was driving uphill on 4 lane highway when I noticed speed dropping, I had dropped to 50 mph, then started to hear metal banging, I was starting to head to burm when some very loud metallic bangs happened, and one or more wheels locked up and sent me into a spin to the right, and I hit gaurd rails. I was on rt. 68 eastbound in maryland, on sideling hill. After having it in the garage the mechanics told me that a gasket or seal leaked on the differential, they told me they have never had it happen, nor do they no why.