Yes that would be first thing to check Scanner would tell if operating need engine , 2 0r 4 wheel and trans to locate

Yes, I'm sorry. Again I have a 1998 Dodge Dako reg. cab with 2.5 ltr. 4 banger, 5sp manual trans. and the ABS Brake light are on....The speedo will work or come up until about 30 to 35 mph either direction speeding up or slowing down. I had it scanned with a scan tool from O'Reillys Auto Parts and the codes were engine codes (no ABS codes were stored) one was a catalyst fault like a catalic converter and the other was a pwr. streering pres. switch. The check engine light is not on......So that's weird, another thing that is weird I told the guy to just clear the codes to see if they would come back at a later date and he could not clear the codes stored the scan tool would not let him, the message on the screen said function to clear codes failed. So which sensor should I replace first? Does this truck have a VSS in the trans housing next to the flywheel if it's a standard trans.? My gut tells me to start with the ABS speed sensor mounted on top of the rear diff. Or is it one of the wheel speed sensors?

Thanks for your help.

it is real common for these vehicles to have the Issue with the rear speed sensor located in the differential. What I usually see is an open in the coil . It is a Hall affect sensor the tone will is actually mounted to the differential case assembly...in some cases the tone wheel may be damaged from an internal failure in the diff but most of the time it's just the sensor.