Here is a little more info. I have conflicting survey info on this area so this info may not be exactly correct but it is pretty accurate. As Bob said it is the Sewell Gun Club Rifle Range.
Fruitland was a maze of properties in that area and over the years Wharton and the state has acquired most of them. However, there are two that are still privately owned and you can actually see them on Ben's map.
http://maps.njpinebarrens.com/#lat=39.73333006320139&lng=-74.7004222869873&z=15&type=m&gpx=
There are state monuments and a few stones on those property lines. Here is the most accurate info I have. I do have info showing much larger private holdings by the gun club but I am not certain if that is true.
So, a little challenge. Lets see who can be the first one to photograph the biggest stone and the nearby state monument on those property lines. So which one is it you ask? I can tell you that you have driven right by it each time you travel down Quaker Bridge Road. And you can even see it from your car if you know what to look for. So the next time you are out there keep your eyes open and be the first to post a photo of it here. Well, the second, I already have posted it years ago
Guy