Brake question on a 1992 ford explorer 4 wheel drive?

My brakes on the front of my car have been rubbing or sticking from time to time. I have lock in hubs and when I lock in the hubs the problem worsen. Then when i push the button on the inside of the car to go to four whell drive it worsens even more. So I first replaced the brake pads, secondly replaced the calipers thinking they were naturally bad, I am thinking it is the master cylinder but don't want to buy more parts until i know whats wrong can anyone help me.|||Im thinking you have a bad wheel bearing or ball joint putting the rotor into a bind, and when you put it into 4wd that really locks it up