1999 Isuzu Rodeo Question Front pads good, back worn

I recently replace my brakes. The front pads looked good. They looked to be about 25% worn. So I put them back on. The rear shoes were pretty worn down, almost to the metal, I would say about 95% worn. I bought the vehicle new, and the shoes and pads are the originals that came with the vehicle. Also, about a year ago, my ABS light came on and has been on ever since. My question is; what is causing the front to rear difference on the pads/shoes? Could this be something with the ABS system that is doing this?