It could be off, but if one side is maladjusted, that makes both sides off. I tend to doubt that an incorrectly adjusted 4-bar could just cause the center of the tire to wear unless the tire is over inflated. 30 miles is a fairly short distance to start wearing rear tires, especially rear tires. Are you sure that the axle housing ends are welded on squarely?
The 4 bar adjustment is the easiest place to start and it wouldn't hurt to double check it anyway.