for the front sensors, s/b right behind the rotor and under a small tab that is part of the dust shield,at least on my '01 2500, easy 1 bolt removal. I cleaned the sensors themselves and the tone wheel which takes a little patience since the access to the tone wheel is small and I went completely around the circumference. The rear sensor s/b in the rear dif on the top as it is on my '01 but I'm not sure on the '98.
I wish my ABS light would go out. It has this thing when it hits -20F it turns on and stays on forcing me to take it to the dealer to reset, altho one time it was a bad rear sensor. I like the idea of having 4-wheel ABS but it has been nothing but problems for me.