1998 Dodge Dakota Brake light & ABS light stay on &

When I started my truck today my brake light & ABS light on my dash stayed on. Also my speedometer was not working at all from 0 to 20 miles per hour (mph). It appears to be reading properly from 20 mph and faster. The transmission seems to be shifting properly at all speeds. I was told by a service station that it might be a speed sensor. I drove my truck over to an auto parts store and they hooked up a scanner to the computer on my truck and found no codes, probably because the "check engine light" was not on, only the ABS & brake lights were on. I then went over to a auto repair shop and they looked for codes and they told me that the codes they found only indicate a place for them to start diagnosing the problem, that the codes do not tell them the exact problem. I do not know if I believe that or not. The shop wanted about $85 to diagnose the problem. Where should I go from here?