I have a 93 bronco xlt and I'm not sure what's wrong with and I was hoping someone could help. When I engage the 4x4 I get really bad grinding scraping noises coming from underneath the truck when I move. I'm running mismatched gears 4.56 front and 3.73 rear, this is how it was when I bought the truck. My 4wheel works but the front tires lock up time to time. I've taken apart the front axle and there doesn't appear to be anything wrong inside so my next guess was the transfer case. Can anyone help me?