Could have been the mechanics fault.
was the rear end fluid changed at respactable intervals? how many miles on the rig? You sure they even DID anything to the rear end last time, or just swaped the fluid? Its kind of odd to swap gears and go bad alreay, unless they used a set of used ones off another truck!! Anything is possible now days, i would def talk to the manager and find out why the failure and what EXACTLY did they change, also talk to MOPAR and find out why the new part failed already. Oh yeah they should warranty you the part and labor, for such a big job IMO...again talk to management. No dealer is alike nowdays.
I had a Dodge Ram that same thing happened , if everything is not installed properly it will make the dreaded noise in my case torque specifications weren't exact. Human error that could be the culprit.My guess......techs fault. didnt take the time to line everything up properly.