Chris, I bought one for my '96 Cummins at NAPA, I don't know if they changed any by '98 but I would think that they are the same.
The NAPA part # is 0p6670 made by Echlin. It takes a special socket that is deeper than the standard size, I don't remember the exact size, something like 1-1/8", but I have the part # for the one that fits the Echlin pressure switch. It is a KD tool. Numbers on the socket is, K-D 3458 USA.