I asked the same question a couple of weeks ago. Here was the response:

There are two stich-welds on each end of the rear main cross sill...one on the top of the sill and one on the front face. With a cutting wheel in a die-grinder, they can be scored and the bed sides can be removed from the rear main cross sill.

There are also four shoulder bolts on each end of the rear main cross sill, securing it to the bed sides.