I've been looking and analyzing the wiring diagrams for hours. Comparing, thinking, contemplating... seem to have a good grasp on how the systems work together. I can't begin to guess how the 4WD computer actually reads all the sensor inputs and stuff to determine everything is working properly...
With all that said...
You have to bypass the actuator controls WITHOUT bypassing the sensors. (so you cannot simply just ground out anything or hook two wires together, a litle more complicated than that). Since the power/polarity to the locker actuator is reversed to disengage the locker, I would think a rocker switch would work fine. Similar to the "05 Frontier locker mod" I saw posted around here somewhere. However this does leave a few unknow variables... You can look at wiring diagrams all day, but that won't tell you how the ECU, 4WD CU, and other onboard computers react to the input from the different sensors. If you engage it manually, Does the computer read the indicator switch and tripp an error since it only supposed to workin in 4LO? Or does that only apply to the actuator circuit? What about other sensors configurations? Does it check the actuator? the 4WD switch position?
The 4WD CU itself is what limits the actuator to 4LO. So even if you can lock the diff, I don't know how the computer will react to the sensor inputs in any mode besides 4LO.
Assuming it works...
It would have to be engaged manually with a switch (and disengaged the same way). Only problem with that is, if it doesn't go in right away, you won't have a computer to keep trying automtically until the indicator switch tells it it's in. You would have to keep pushing it or hold it until the indicator light comes on. The ABS, VSC functions are just controled by the ECU and the indicator switches. Leave all the sensors intact, and your ABS, VSC, etc should still disable as normal. Also by putting in a manual switch, you would not have the safety features enabled to keep you from accidentally engaging it while at highways speeds!
In short, if you bypas the computer, you won't be able to use the factory Diff lock button. You'll have to use something else like a rocker switch (or wire up 2 of them...one to engage, one to disengage.) And pray the computer doesn't spit back errors at you. You'll only know by trying it. If I had my truck I'd do it myself. But if someone wants to try, I'll provide any input I can. However, you must have a full understanding of what you are doing since I will not be held responsible, for any resulting damage or error.
Alternatively... a more expensive, but factory-look solution came to mind....
The Prerunner uses a different Control unit than the 4WD models. However it's still called a 4wd control unit... Imagine that!
and controls the same safety functions the 4WD model does when the locker is engaged. (disables ABS, VSC, etc).
The prerunner engages the locker regardless of 4WD mode, because it doesn't have it, due to a different control unit... DuH! I know you knew that.
The 4X4 can engage 4WD in any mode WITHOUT the locker just not with it. Again you knew that...
Why not have 2 computers? One from the Prerunner, wired up to the locker. And the other 4WD ECU to control the drive mode. Completely disconnect the e-lock switch and actuator, and indicator switch from the 4WD CU (don't want to confuse it) so it NEVER senses that it's active, and hook them up the the Prerunner CU which will send any neccesary info to the other onboard computers. The 4WD computer should work fine on it's own without the e-locker connected.
The trick is, the Prerunner CU needs to be sliced into some of the same wires that the 4WD CU uses to communicate with the ECU, SKU, ABS, etc. But you don't want each CU tripping sensors and stuff on the other one. The solution would be to use diodes to isolate each computer from tripping the other one, but still using the same wires to transmit signals to the other onboard computers.
Personally I like this idea better, since it still enables you to use the factory switches, and keeps all the safety features intact to keep you from accidentally engaging soemthing you don't want. Would be quite a bit more expensive since you have to buy a second computer from a Prerunner as well as the wiring harness to plug into it. I haven't checked and have no idea what another 4WD computer would sell for... But I know I could get it to work.
Anyone else have some input on this??