My 2006 expedition will not engage 4x4. It will go in A4x4 but when I try 4H it does nothing and when I try 4L it keeps telling me to put it in N and press the brake. I have done this about 50 times with no luck. I have read a lot about the TCCM and the shift motor. Can anyone tell me which one to replace first or check 1st from the problem I have stated. Thanks for the help...