How's the speedometer behave, particularly below 20MPH?  If it's also
erratic, the most likely cause is a failed rear wheel speed sensor.  It's on
top of the rear differential, and comes out with a single bolt.  It will
cost under $40 for a new sensor, and should take about 5 minutes to change
out.

If the speedo. isn't erratic, it's still a posibility that the RWSS is
bad/performing poorly.  It could also be a failed front wheel sensor, low
brake fluid, air in the ABS pump, an ABS pump/controller failure, etc. (all
this is assuming you have 4-wheel ABS).  Unfortunately, the ABS system is
proprietary, and most shops don't have the equipment to diagnose it
(directly, anyway).  That's something only a dealer can do.

On a '98, it's possible to disconnect two of the three wheel speed sensors
at a time, and use the speedometer to see if they're good.  I've done this
before.  The rear sensor disconnects on top of the rear differential.  The
front sensors disconnect at a gray connector on top of each front inner
fender panel (follow the wire next to the brake lines coming off the front
calipers).  Drive the truck with one sensor at a time hooked up.  If one of
them fails to allow the speedometer to register, you've found the bad
sensor.