Toyota front axle would need to be flipped to run driverside drop. The transfer cases are stout but gear options are limited. If your truck is an auto you can bolt up all the popular t-cases (6 bolt round 23 spline) if you have a manual swap in an auto from another Montero. The automatics that come in the Montero are really similar to the Toyota and Jeep Aisin transmissions except the Montero ones are a bit beefier and early models 87-92 are hydraulic controlled and don't need a shift controller.

I am pro SAS and I think you should swap in a solid axle up front BUT you also haven't run one of these before so I suggest you stick it out for a while and see what it can do.