New pinion and ring

Bought a used 2014 4x4 with approx.160k on it and it has whine starting around 40. It bugs me the most around 40 50 when "burping" the throttle but it's noticeable at highway speeds as well. I decided it's going to bug me to much to so ordered a new ring and pinion assembly.
A dealer is installing the unit so I mentioned I'd like to keep the old one. He says they normally just throw them out but I was thinking if I had to I could always reinstall it if something happens to the new one. Am I crazy to want the old assembly back?
The reason I went with an assembly is they said it is just as cheap as replacing gears?

 

Ended up keeping the used assembly.
Thought I'd mention because 8t was the issue, no whining(howling) and any speed now.