One problem related to differential unit has been reported for the 2002 Mazda B4000. The most recently reported issues are listed below.
Since I purchased the vehicle there has been noticeable play in the rear differential. I ignored it for a long time and then after the warranty had expired I took it to a repair shop in the area that has a very good reputation for repairing and setting up differentials. After examining the gears in the rear end he told me the play was caused by a part that was machined incorrectly at the factory. He said it was a hole that was drilled too large for the pin that is installed in it to hold the axle gears in place. He assured me that it was not caused by wear because this pin is not a moving part. I have continued to drive the truck without any repair being made, it does not seem to be getting any worse, but who knows how long until it fails?.