The Ford 550 is a dual rear wheel rig but this rig uses only the outer wheel of the pair on the rear. I wondered idly whether that puts an awkward stress on the rear wheel end.
It's a wider than normal wheel and tire (a "super single"), so wide that it could not be used in a dual pair... but it still has the offset of a dualie so it fits properly on the front. When used on the back, it places the centre plane 6 or 7 inches too far out relative to the hub and bearing, which seems like a bad idea to me. It also leaves the brake disks hanging out in space ready to bash a rock, which looks bizarre when you get an angle which shows the disk.
I asked Earthroamer about this in an e-mail message (more nicely than I put it above), and they declined to respond. My guess
is that their loading (limited by tire capacity and by room to carry stuff) is so far below the axle's rating that even this improper use is reliable.