Based on the location, I would guess its dry skin. Your choice of bedding is probably adding to the problem. I would recommend using mane n tail (horse conditioner) on the effected areas the last thing before you put the steers in the cooler or out for the night. It will soften the skin and help the hair grow back. It works best if left on overnight or so and then just rinsed out the next day. By the way, I would check their knees too, if the hocks are dry and bald, usually the knees are too! Good Luck!