You need an oscilloscope to really be able to read the output from the rear diff sensor.
Most problems in that area are with the connector not making good contact on the sensor, the cables nearby chafing onto the parking brake cables or on the chassis somewhere or a broken wire.
The front ABS sensors likewise could have cable damage.
I had a no speedo issue with my 2002 Export CRD...no ABS fitted. Turned out to be a bad connector fitting on the diff sensor. This set the CEL light ON and speedo would go up to about 30 mph max no matter what speed I did.
Once I fixed the connector the speedo worked properly but the CEL light stayed on for about a week....took a couple of start-stop runs for it to reset.
Best to download the correct year Jeep KJ Service Manual here....includes the Wiring Diagrams in Section 8W. I would start with tracing why the Emergency Brake light is ON all the time....may be related to the other problems....then physically check the speed sensor wires for breaks/shorts to chassis. The wires probably disappear into a larger harness where it is hard to physically trace the wires further.
I had about 9 wiring errors on my Jeep after a frontal impact.
The main harness on the KJs goes all the way around the engine bay like the Washington DC Beltway....going through many connectors in the process and on my Jeep going through a solid sqaire metal tube below the radiator...where the damage was done. I had to completely isolate each faulty wire one at a time...disconnecting both ends....including splices...so that the wire was in theory hanging in "mid air".
If I could not them measure continuity from end to end with an Ohm meter....or if I measured a partial leakage to chassis...I would physically cut off all ends of that wire and tape up the ends. Then I replaced this wiring with new wiring...soldering the ends together and using shrink-tubing insulation. I did not try to follow the long round-about journey of for this new wire but took it point-to-point via the shortest route. Then marked my changes on the relevant wiring diagram pages and left it in the glove box for the next Owner!
The Wiring Diagrams will show the relevant wiring colors, splices involved and connectors in line with the route of the wiring. You will need a cheap digital Multi meter.
Also check that the speedometer is actually moving by holding the Odometer reset pushbutton IN while turning ignition ON. The cluster lamps and Gauges should all light up or move around. This is not a diagnostic and does not return error codes.
Jeep KJ Service Manuals:
Index of /manuals/Jeep/KJ