Adam Shipwreck Nagle

I know that this is a common problem with these trucks. I have a 2008 Lariat, when it's in 2wd, I hear a humming noise coming from what I believe is the driver side. I replaced the hub and bearing assembly, IWE, both on the driver side and the vacuum assembly. I also replaced the unit on the firewall. The problem is still there, I did notice that when I disconnect the electrical connector on the firewall, the problem goes away. Also, it will not engage in 4LO, only 4HI. Could it be the switch on the dashboard? That is the only thing I have not replaced. Any help would be much appreciated.