I would guess its the manufacter covering thier rears in case a diff breaks.

 

Flent, When I did have a 4X4 I changed my front and rear at the same time. The only reason I changed them was due to allot of mud riding. I had a Z71 that after a few deep holes sometimes it would get water in the front and rear even with the vent lines ran up high. To me itwas and ischeap insurance plan and simple......

 

I think 15K or 30K or even 60K is overkill. We run class 8 big trucks and their rear end oil gets changed at 500K that's on factory fill synthetics. The 9 inch Ford rear end in the 1966 F100 my grandfather bought brand spankin new and used on the farm for 3 decades still has the factory fill diff lube in it. And that was back when there was no such thing as synthetics for automobiles. They were putting dino in the diffs[:-] I did change mine to amsoil svg at 7500 miles, and there was some metallic breakin "mud" on the drainplug magnet, but I intended for that to be the last oil I ever put in there.

 

i am the parts manager at a dodge dealer that sells a lot of trucks, every rear i see getting worked on didnt have the fluid changed.
dodge nor any other manufacturer would overkill the maint. it would raise the average maint cost of a vehicle. that is one of the figures
fleets look at when purchasing vehicles.i just change mine and be done with it.