One usually sees that done to the 2-1/.2 ton trucks. This one is a 5 ton. The main reason for removing the rear axle and shortening the frame is so that you only need a regular license to drive it. The third axle means you have to get a class 3 (in Manitoba, I think there are different classes in other provinces). They usually use the cargo box from a 1-1/2 ton trailer, which is what this one appears to have.
I have a couple of them myself, but I prefer leaving them original.