Rear diff dilemma

[Stan the Man]

evening, I have a 2018 Sierra 1500 4x4 w/6speed tranny, was hit on the right rear,between the tire and rear crew cab door at 4K miles. Dealer replaced right axle, wheel bearing and seal  ring and pinion set up, had to fight insurance to replace the entire housing, but they agreed. Now at 24k miles and the rear diff sounds like my Ford Super Duty 450 w/ 350k miles, it’s howling on deceleration . Took it back snd dealer says oh a shim backed out, they said they replaced ring/pinion set up, said everything. I questioned if wheel bearings and outer seals were changed, nope, 20 year tech said didn’t need it. My long winded question : shouldn’t every bearing and internally lubricated part have been changed? I asked for a sample of the oil for an independent evaluation at Napa, but they say they mistakenly dumped it all into the reclamation vat. I haven’t picked the truck up yet. Any advice?  Repairs are being covered under original Powertrain warranty- this time. 
and my apologies if I’m posting in the wrong venue. 

[CamGTP]

When it comes to warranty work like that, they are only going to replace what is causing the problem.