I have a roaring noise from rear of my 2016 Nissan NV3500 with 100,000 miles.

Hello, as this noise still occurs in neutral, I'd lean towards axle bearing or even abnormal tire wear pattern. Check those tires first -- are they unevenly worn or have high and low spots? Other than that, yes, could be an axle bearing on the opposite side. Too hard to know without hands on inspection. It would be probably accurate to replace the side most likely to be the source of the noise and maybe more noticeable. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops