This modification does not have anything to do with the CENTER DIFFERENTIAL LOCK SWITCH (CDL) directly. You do not have to do this, if you just install a CDL switch. The purpose of this mod is to overide the default behaviour of the vehicle that automatically lock the center diff in low range.
However if you do this mod, you are going to need a CDL switch, otherwise you will never be able to lock the center diff.
Remove the dead pedal and the left front sill plate.
In the left kick panel you will find a Silver metal box. That is the ABS ECU.
Remove it, and behind you will find a black plastic relay about the size of a cigarette pack. It says "transmission control relay" on it. That's what you are after. The EWD calls it the "center diff lock relay", same thing.
Unplug the harness from it and take a terminal pick and remove the black wire with blue tracer from pin#7.
Insulate the bare terminal to suit and then put everything else back where you found it.
If you have the low 4 position switch at the transfer case unplugged, crawl under there and reconnect that as well.
What you have just accomplished is you have interrupted the signal from the transfer case that tells the lock ECU that you went to low range but you are still sending a signal to the transmission ECU telling it that you ARE in low range. Therefore the transmission will shift in the correct low range pattern. In addition you now have true manual control over center diff locking. It only locks when you tell it to without regard to transfer range selection.