What Would Cause This?

I recently purchased a 2014 Titan that came with what appears to be a Rough Country 3" lift up front and a shackle rear lift. One thing I didn't notice right away but am noticing now is that the bed of the truck sits at an angle where the rear side appears to be tipped upwards. When I measure the gap between the bed and the cab, the bottom of the gap towards the ground is 1 3/8" and the top of the gap is 5/8". I've had multiple people look at this including the Nissan dealership and no one really knows what's going on. The frame doesn't appear to be bent, but I'm wondering what else would cause the bed to be slanted like that. It looks like the prior owner could have potentially backed into something that lifted the bed up, but again I have no idea how it would do that without bending the frame. Any thoughts?