I've seen people do it, but they use the smallest, lightest slide-in campers out there (as well as add rear sway bars, stiffer springs, etc). Lance products are for full size trucks.
as to the legality, I've got no idea.
still kicking myself because I didn't grab various things over the last 20 years. most recently the flywheel when I got that ax-15 because I was in a hurry and simply forgot.
keep in mind that pulling the rear shaft can allow fluid to escape the t-case. I've seen where guys unbolt the rear shaft from the axle and then use a ratchet strap to hold it up to the underside of the truck