One problem related to differential unit has been reported for the 1998 Dodge Ram 2500. The most recently reported issues are listed below.
When backing up a clunking noise emitted from differental. Dealer replaced fluid and added some "additive". Did not correct problem. They kept truck to disassemble differental, found nothing amiss, replaced fluid and additive. At this time, all seems ok. The cost was $83. 90. When they decided to keep the truck over night they thought it would be covered under warranty. However, it was out of pocket expense.