If i am not mistaken, sometime in 2010, they shut down the rear end plant(s), retooled for the rear ends that were designed by ZF. Likely they used up all the rear ends that were made before the retooling.
Rams now have a proprietary ZF axle which weighs around a third less than comparable axles
The proprietary ZF axles weigh up to 34 percent less than, and improve fuel efficiency by 2 percent relative to, comparable axles. We began to incorporate the ZF axles in the Jeep Grand Cherokee and Dodge Durango in 2010 and will use it in the Ram pick-up truck starting in 2011.
Chrysler news announcement with ZF