I'm in the process of doing a 2004 SR5, not much info out there so ill post, there are 2 solenoids next to the air box, the one closest to the front of the car engages 4WD by vacuum from the diesel pump, the pink wire with a bluey/green stripe can be traced back into passenger side kick panel, cut and re-route that wire through a toggle on the dash and your good, a relay could be installed but pink wire is fairly small so the current draw is minimal.
Hint....use a switch with a seperate bright led circuit so it let's you know when your front wheels are not being driven as it goes into 4WD automatically when in low range.
Makes backing the boat or caravan so much easier, especially on a hill.
Cheers Blair