Is the car sitting level - sounds dumb but check the frame at the same place on each side.

Are your wheels the unilug type with an elongated lug hole that fits both the Ford and Chebbie pattern? I had trouble with a pair of these, 15x10s with 12" tires plus being wire wheels, several years ago on a Ford 8" diff and had what appears to maybe be the same problem - they hung on the axle flange until they were properly centered - a PITA (also to keep clean) so I got rid of them