What is the "shelf mod?"
The "shelf mod" is a form of shelf or netting, suspended underneath the ceiling in the rear of the truck. It can be made from regular shelving yourself, using a little ingenuity in mounting to the ceiling hooks... Or you can get an aftermarket product such as Raingler nets.
Under no circumstances should heavy objects be stored in this location. Strictly meant for soft or irregular items such as towels, jumper cables, tow straps, etc.