Differential Service

He said when the particulate matter in existing fluid will, when replaced, make the seals/ gaskets the "weakest link" & they may fail more readily. Let a sleeping dog lay.

Why/How would new fluid cause problems ?

I ran into the same question when doing other fluids at 60 and every interval after. Shop said they could but new fluid could actually cause more harm by being pristine, then leading to other issues, particularly before heading out of a trip. Thanks for raising the topic for more perspectives.

Should it be? Absolutely. 50k on an axle largely operated at max capacity, it’s due.
However realize “servicing” is nothing more than dumping the oil and replacing. So it shouldn’t be very expensive if it’s already on the rack.
$100 in fluid and an hour of shop time. Any more is overcharging.
Also be ware that metal shavings isn’t necessarily bad. Metal chunks are. All axles lose some material through operation, that’s why the magnet is there, to collect it.

Can’t help diagnose the noise but it’s possible it’s the rear end.