1998 Dodge Ram 1500

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.

Mine did the same thing on my 97 . It was low brake fluid in the reservoir, caused from a leaking brake line. Your fluid will also go low as your brake linings wear.