What goes bad first in a rear differential?

[The motor guy]

I'm at 247,000 on my 11.5aam rear end. This one has actually been maintained, so come rebuild time, do you always have to do a full rebuild or can you just put a couple new bearings in it and save a bunch of money? It's not giving me any issues. Makes me wonder how much longer I have before it gives up the ghost? This is my 2007.5 dodge cummins 6.7

 

[ripcord]

My cousin in law put a million miles on a 2 valve Cummins driving a rural mail delivery route. He never mentioned working on the rear end, just a transmission.

 

[Oillover]

The fluid.

 

[Speedbird001]

I would guess a seal > leakage > low fluid > higher temps > donski

Couple times I've had diff problems it was some kind of harsh sounding clunking, especially when turning. I was smart enough to take most of them in straight away for a relatively low cost repair. (With the VW I was 20 and broke... rode it out another 6 months and it was then unrepairable and required a full rebuild of a used unit)