If I've followed this correctly, here's a summary ...
- OCIs every 30k miles or so since new. This would imply that these are not residual metals; these are being generated with substantial frequency.
- No noises or metal on magenet.
- Cover removal and inspection pending. Please take pix and post them up.
- This rear diff is not easily serviceable due to parts availability? But they can come up with a replacement? That seems odd. What year of truck?
Rock Auto has parts for many years of this truck. The 2015 model shows a complete rebuild kit available, but no ring/pinion. However, I'm sure any reputable reman/rebuild center can source parts from other places. For example, the following show parts available:
- Rockauto
- nissanpartsusa and
- nissanautomotiveparts and
- nissanpartsoverstock and
- Differentials.com has a lot of parts listed also.
Maybe they don't have every part you need at just one store, but I expect it would be easy to source what's needed, even if you had to visit a few websites. Without knowing the exact dets of your truck (engine, drivetrain, etc), I cannot say for sure, but it certainly appears there are plenty of repair parts available from multiple sources.
What's the alternativre? Sourcing a used axle from another vehicle? How do you know it won't have the same wear issues, presuming it's a similar/same year model of component as yours? I'd rather have a reputable shop reman my own axle and know it was done right than buy a used axle with unknown history, but that's just me. Not that I'm advocating a rebuild yet. But to say it can't be rebuilt is perplexing to me.
I guess this comes down to the topic of inconveniences. If you just continue to drive it, it may (or may not) leave you stranded somewhere. What's that risk worth to you? If it's no big deal, I'd just drive it and not worry about it. If being stranded is a big deal, I'd advocate for getting the unit remanned.