My first MJ was an '86 Metric Tonne 2wd model. 4 cyl (with TBI), 5 speed with a D35 rear axle (it may have had a D44 originally) With 4.10 gears and 235/75/15 tires, I drove from Seattle, WA to Ouray, CO (by way of Moab) and back in '02. On the way there, I decided to check what my GVW was. 6,000 lbs according to gov't certified road scale!
This was with 3 adults, a canopy, 2 spare tires (was I glad to have those! We were hit by a CSK delivery truck in CO, which misaligned the front and cost me all 4 of my tires, due to being rotated to the left front every couple hours on the way back) and a pile of spare parts and tools. We made the trip there and back and only offloaded a few bits and pieces along the way and ended up bring more parts back with us. Aside from death wobble (first started in Idaho) and the slave cyl for the clutch going out (kept filling it whenever I had to stop. 10 pushes later it would be empty again so I did a LOT of synchro shifting!) We averaged 70-80 mph across the desert and dropped to 45 in the mountains.
Putting a 5th wheel in the box would increase the load capacity if you did a little extra frame beefing up while you were at it. (and a heavier axle of course) There was one hauling a 50,000 lb 5th wheel semi trailer posted on the MJ list a year or so ago.
