1999 GMC Jimmy ABS light comes on, hard time stopping

My ABS light is coming on, sometimes when trying to stop, the truck reacts as if using the ABS system to stop when on ice/snow.

Took to mechanic, said rear brakes replace, rotors are rusted & pitted. Does this sound logical and if so, could you please give me your best guess as to the total costs? Maybe my son can do this.