I have a 98 Explorer that makes a terrible knocking noise when in 4 wheel drive. I have had the transfer case and tranny completely gone through and it still does it. It will also do it in 2wd if I accelerate real hard. I disconnected the electrical at the transfer case and the problem went away. Of course I don't have 4wd but it solved the problem in 2wd. Someone suggested that a module was bad? Is there a reset? I was also told that because I had new tires on the front and old tires on the rear that the sensor was sensing different speeds between the front and rear. I put 4 new tires on, it didn't help.

 

Maybe something wrong with the shift motor?