For 4 low as you know the truck must be in Neautral.
Does your tranny have a neautral safety switch?
If it does you need to hook up the electronic shift wire that is coming out of your firewall (main harness, drivers side, near brake booster) to the neautral safety switch...
Then the elctric touch drive should work again.
HOWEVER I willhave to do some looking up on this with my diagrams at home, because I believe the computer is hooked to the neau swich, then the computer tells the electronic 4x4 its okay (the EEC-IV actually has a electric shift output, even the Mustangs believe it or not have this pin...hehehehe)
OR we can find the wire coming from the shift controller (Box) and hook it up to a dummy switch, so it would be up to you to fool it into thinking its in N. (similar to what you were thinking)
The other option is to bypass the controller (Box) and figure out the wiring going to the motor itself. I mean its a motor with like 8 wires, and I know it has a power (+), ground (-), signal for neu, signal for high, signal for low, and a 2wd.....
So it is possible to send either a 12V + or a 12V - to the correct wire (series of manual toggle switches, relays, 3 pos switch, whatever) to make the shift motor move.
Last oiption is to ditch the electric T case and go manual, but personally I love the touchdrive and after 245K miles on my 88 I have only cleaned the shift motor once, it works perfect every time, like new.
Now email me what you are thinking and we can get this working.
Also what year 351 do you have in there?
We can get your tach, oil, temp, cruise control, everyhting working possibly depending on your 351 and how far you want to take it.....
jamiefortune@lerchbates.com