On a full-floating hub, the axles just provide torque. They support nothing. If nothing inside the pumpkin is turning it won't need lube. At some point axles like this had shafts with splined flanges to drive the hub. Then there was a light gauge cover held on with like 5/16 bolts. I don't know how late that system was used but if yours is like that pull the axles, grease the bearings and put the covers back on. You're done.
If you fill a pickup bed with rocks you'll need trailer brakes. What are you going to tow it with? If I understand what you're doing, you'll have to run regular 20 or 22.5 truck wheels to match the hubs with tires to match. In our state anything over 4,000 pounds has to, not only have brakes, but have a brake-away system too. There is made a "reaction brake" system commonly used on boat trailers (electric brakes don't like salt water). You could do a little figuring and come up with an appropriate-sized reaction-brake master cylinder, then plumb that into the existing brakes. I wouldn't pull such a trailer around without brakes. Even your spare Peterbilt tractor will skid the drivers with enough weight pushing it.
