You should probably look for an exploded diagram of the rear axle to see how the axle tube bearings are set up. They just about always have a bearing race in the housing, but some axles use the axle itself as the bearing surface, while others use a race. If it rides directly on the axle you may want to do much more frequent fluid changes and check it from time to time.
Also as an FYI, my '15 has a payload capacity of only about 1100lb, you're very likely way over what the truck is considered rated for. I occasionally way overloaded my Hardbodies over the years (1600lb worth of canned sodas for a sci-fi convention on my '89 comes to mind) but not full-time. The extra leaf might well address suspension issues, but I believe that you still have a fairly thin C-channel frame not much different from the one that my older trucks had. It really might not be up to the task for the long term, or if you're in a fender-bender or some other reasonably common stressor, the frame may give way.
Because the Hardbody was sold on the International market as Nissan's go-to truck they did have a heavy-duty version, one variant was sold as a regular cab with an eight foot bed, another was a cab-and-chassis with dually rear wheels. I believe both were considered one-ton trucks, but they might not even have gone as far as king-cab models, let alone made it from the D21 chassis to the D22 chassis.