There're 2 common issues with these ABS systems (besides the obvious things like torn wires from off-roading):
1) the 12V wires supplying power to the ABS controller and pump develop a bad connection somewhere in the Power Distribution Center. These are 2 thicker red wires going to the ABS controller. You need to check the resistance of these to make sure they are good. Simply checking the presence of 12V is NOT good enough.
2) The pins in the ABS connector/module loose (springiness) contact. The best solution I found is to solder the male contacts (of the ABS module) to make them fatter. You have to be GOOD at soldering!!! You have to cover the whole length of the pins UNIFORMLY!!!
I keep planning to do a write-up on this, just not enough time.
Get the 98 FSM in electronic format, this will have the right info for your ABS system. If you don't know where to get it, PM me. You CAN fix this yourself, but it's time consuming and you have to be good with a digital multimeter, soldering, etc.