Here's a thought... take the box off the frame... build a proper tube or box frame of your desired length... with deck... and place a heavier duty axle and leaves under it after determining oh, say, 60/40 front-rear load bias, laden. Just temp mount the axle till you have the sweet spot nailed.
And there you have it. Nakspedition style.
Heb Dduw, heb ddim; Duw a digon
I'm that gun-totin', farm-raised, evangelical, pro-environment, OHV ridin'/drivin', Southern civil rights pro-labor Liberal yo' momma told you couldn't possibly exist.