All right, this has me really curious so I found the Haynes manual.
It shows 2wd hubs to look just like mine.
It shows the typical 4wd hub (with the axle splines sticking out and the bolts for the free-running hub) but has a caption that reads "On Pathfinders, Xterras and 4WD Frontier models, remove the screws securing the lockwasher...".
It doesn't show a hub like yours, with the screws and lockwasher (they call it a "locknut" in the next frame), but, given the Xterra picture you provided: Is there any way that your truck got Pathfinder/Xterra hubs on it either from the factory or from a repair shop?
And yes, they say "unscrew the locknut with a tool like the one shown here (sort of like a cross between a couple of spanner wrenches and a pair of pliers), or a special socket".