From your pic it looks like the you removed the screws for the shelf supports. The shelf itself is either nailed down or glued down to the supports. If you have enough room, I'd start tapping the shelf in the corners with a rubber mallet or hammer from the bottom.
'16 Ram 3500 MC CTD Dually, 4:10 rear
'14 Ram 2500 MC CTD (gone)
'14 287BHBE (gone)
'16 North Point 377RLBH (current and hopefully last)