@LightScout03.
I believe that on the 2005 the speedo needs both the output from the diff sensor and the ABS sensors to work.....so the ABS sensors may have an issue!
You have changed the diff sensor but for general information there was a TSB issued years ago by Chrysler that all 2005 KJs had their diffs filled with the wrong oil. This oil chewed up the diff sensors. The fix was to take the Jeep to the Dealers who would drain the rear diff oil, clean out the diff, fill with the correct oil and fit a new sensor.
The only way to test the diff sensor is with an oscilloscope as mentioned.
You would otherwise have to test all the wires going to the diff sensor and ABS sensors with a multi meter to check for continuity and for shorts or partial shorts to chassis...download the 2005 Jeep KJ Service Manuals here...section 8W has the wiring diagrams and the Service manuals have some theory of operations information on how the speedometer operates.
KJ Manuals:
Index of /manuals/Jeep/KJ