The causes could be bent frames, bent k members, bent steering box mounts, bent torsion bar mounts, bigger t-bars, different rod ends, or different steering components. It isn't something that could be easily pointed out electronically, but would need to be put on a frame rack to check. Slight variations from one side to the other aren't unusual, but an inch and half difference is substantial.
Slight variations can be altered by shimming the steering box or modifying the idler mount. Differences as large as what you have are a different story though. Do you have a longer pitman arm? That may cause this. You may also want to do a cursory check of the frame structures. Somewhere on here is a layout of points to check a chassis for squareness. In simplest terms, you want to check diagonally from fixed points to see if there are variations on your structure. Any significant differences you may want to be verified by a body shop on a frame machine.
