All four tires cupping on the inside tread
 

I'm not sure if the title makes sense because I've never seen it before. All four of my factory BFG tires are cupping on the inside of the tire tread, not the outside where you normally see it. The fronts and rears are wearing at the same rate, but maybe that's due to being regularly rotated. I was told it is common with 4x4 vehicles and I should rotate them by removing the tires from the rims and swapping the tires side-to-side so the cupped tread ends up on the outside edge. Anyone heard of this? THX