Mine wont drop into 4low unless its under 5mph and has the clutch in and 4high wont engage after a certain speed either. If the computer isn't happy with any of these inputs the lights will flash and what not.
Maybe check were the computer gets this information from and check there? A bad switch could be sending the wrong signal to the computer.
Edit: If you find where these switches are and figure out how to by pass them let me know. I would like to remove mine.