Looks to me like all the possibilities have been listed already, and to the best of my knowledge, it has to be one of those three things:]
(1) differential tire pressure,
(2) bent or damaged arms or brackets - could have even been done at the time it was manufactured, or
(3) mounting brackets installed wrong or slipped after installation - the manual with my loaders said to install the brackets with all the nuts and bolts, with everything sitting on a flat level surface, but to not tighten any of the nuts and bolts until all were installed.
Of course, that's if it's uneven when parked on a level surface. If it only becomes uneven when it starts to raise; i.e., one side raising quicker than the other, then I would guess there could be a problem with a hydraulic cylinder or line, although it would also seem possible to me that some of the mounting nuts and bolts could be loose.