There is a diagram somewhere to make a LED / resistor / switch / banana plugs that will work on all systems ( ABS / engine and so on ) If you have anything that uses an LED you can get parts.

In general, most ABS failures regardless of car seem to come down to speed sensors. The area around the sensor rusts lifting it off the mount reducing signal. The wire flexes and breaks also causing a problem.

I'm wondering if the transfer case is on / off or modulated to engage the front drive shaft? If the electrical system is unfixable i'd be real tempted to use a switch 4WD ON / OFF then drive it like a part time 4wd truck. ( 4 wd off on dry pavement )