The 4x4 box just causes the light on the gauge cluster to come on. It doesn't do anything else. The 4WD is entirely mechanical.

Looks like there's 2 different MFI (engine) relays for the V6. One for 1990, and then an updated one for 1991-1995. You should be able to get the later part number (MB906016) from a 88-90 V6 Montero, or a 91-94? 3.0 Montero. Looks like RockAuto has them for ~$65 if you can't find a used one.