Thanks for digging up that patent mbeach. It sure has alot of information about the logic and control systems of TOD. The techs at Borg-Warner put together quite a system here. And the price of the TOD ecu now seems somewhat reasonable. The amazing part is that they didn't put in any user control, it certainly would have been pretty simple compared to the amount of work they put into the system. (maybe it was a liability or marketing issue)

After looking through the patent it appears that the signal to the clutch is PWM and if we cut that wire we need a dummy load (to satisfy the feedback requirements of the TOD computer) Other than that there might be still be more than one way to skin this cat.

I think I am going to apply my modifications after the TOD ecu only, as I dont want to fry such an expensive component. After studying the wiring diagrams for awhile I think it might be relatively simple to alter the dash lights along with the TOD ecu-solenoid signal.

Heres my plan:
I want to control the sysem with a knob, probably 5 positions like mbeach stated earlier: Auto, 2wd, ~15%, ~30%, and 50/50

I want to create a pin-to-pin harness between the wiring harness of the vehicle and the TOD ecu (to minimize cutting stock wires), this would make the modification very reversable and easy to install for most anyone. (literally "plug and play")

I plan to cut the ecu-solenoid wire with a relay, so when the knob is in auto mode everything functions normally. Then when the knob is anywhere other than auto mode the ecu is connected to a dummy load and the solenoid receives the signal from a PWM controller.

The PWM controller will probably be a 555, or 556 timer set up to send a 50Hz signal. The controller will be voltage controlled, with the control voltage coming from the 5 position switch. (with the descreet voltages set up by finite resistances)

To accompilsh the correct display lighting I was thinking about interputing the stock wires for the auto, and 3 bar lights with transistors, wired so when the knob is in auto mode the lights are controlled by the TOD ecu normally, but when anywhere other than auto, the lights are disconnected from the ecu. (this darkens the auto light, and allows the 3 bar lights to be controlled by the knob position; see next paragraph)

Then another set of transistors would connect each individual bar light to the illumination wire depending on what position the knob is in. (this lights the 3 bar lights up correctly)

And the whole thing could be bipassed by a relay controlled by the 4low switch, so if you took the transfer case shifter out of Auto the TOD ecu would have all electrical control

I have a rough circuit diagram drawn up right now, and it seems to look like it should work. I plan on spending some time in the University's electrical engineering lab tonight or tommorrow setting up the circuit and performing tests. I also still need to look at a few things in the VX before I go too far.