I don't know about sitting it directly on the axles, however I do carry my 20' aluminum extention ladder under mine basically with 'U's mounted to the frame. Many miles and no harm to the ladder. Actually it still looks rather new and shiny...
It was a design that I made a few years back that has an aluminum box on each side, clean side has clean water stuff such as filters, white hoses, pressure regs, etc., and the dirty side which has all of the dirty water stuff. The ladder slides between them, think of it like saddle boxes... I had the guy make a ~24" w. x ~6" tall aluminum "U" to hold the end. No problems. I have some pictures somewhere if you want to see it...
