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Hey Brian, you haven't been tricked by the foot on the brake shift 'feature' have you?
You have to put your foot on the brake with the vehicle stopped before it will shift into low range. Call me a geek, but the only reason I know is cause when I bought my F250 I sat up all night and read the manual from cover to cover. I know it is sad, but hey, if your problem is as simple as that, well it was worth it. I actually took mine out and tried it the next day - that's really the only reason I remembered the 'foot on the brake shifting into low 4WD feature'.
Was the trans loaded up? One last thing to try could be to reverse a little bit to free up any torque build up in the tx case, then back into neutral and try again.
If still no luck I guess it is out with the circuit tester to check the switch is providing the signal and if the signal is reaching the tx - and the connections as per previous post from Jim.