Not familiar with the Rodeo, maybe it only had rear ABS before 1998 but there were certainly plenty of older 1990s cars with the 4-wheel type. Most of the time ABS problems are due to bad sensors, and those are usually due to coax cable wear from flexing. Fronts usually go first due to more cable flexing from steering but I've seen rear ones go bad as well.

Debris or damage on sensor trigger rings can also cause ABS problems.

In general if you have a bad sensor with an open circuit the ABS light will stay on immediately on starting the engine since the computer will detect that right away. If it's a problem with a trigger wheel the light would come on after you start rolling since the computer won't detect the problem until the wheels are moving.

A dealer-level scan tool would really be needed to properly diagnose the system. However it is possible to test the sensors with an ohm meter at their connectors. (ABS sensors are just electromagnetic coils.) Typically they would be about 1K ohms but there should be a spec for your vehicle. Trigger wheels can be visually inspected.

Of course on a vehicle that old it could easily be a bad ABS module.