98 has 2 speed sensors. one on top of diff (called rear abs sensor) and one in the tail housing of the trans (2wd) or t-case (4wd). rear is about $30, trans is about $16. you can look them up on autozone, rockauto, etc.

 

If you have a handheld scanner you can see if the computer sees a signal. If not then you have a problem with either the computer or the wiring.