Have you attempted to check the control signals at the valve assembly? I would back-probe the switches to see if they are getting a signal to be "on." If so please let me know what circuit numbers and I will see what I can come up with.
The diff is programmed to be locked up to 12mph or so, this signal should be present on takeoff than disappear at or near this speed.
Not to second guess the diagnosis but are they certain the front axle is in operation and causing this complaint?