*Kearny* not Newark as reported before? That's interesting. So there goes the whole commuter in Insurance Field with a grudge theory. Kearny is small and very industrial, and not in a quaint way. It's just over the river from Newark but pretty isolated from public transit hubs, although one edge isn't too far fro the Harrison PATH station. It'd be a little more complicated and out of the way to send letters from there since it's not a stop on the usual NJ/NYC commuter routes.
edited to add: in addition to having been an industrial hub, it's a very middle/working class, primarily white community with a lot of families who've lived there for a very long time, stuck smack dab in-between two large cities with a far different population in terms of community diversity. It's urban but small town, if that makes sense. In a way now that the letters are linked to Kearny, I'm personally more open to the idea it could be a much older person who has some psychological issues and who has been fixated on the house for a number of years (even though I don't think they sent any letters other than the one mentioned in the lawsuit to the previous owners).
But who knows!