Stuck/bouncing Speedo and/or Tach, 2006 Trucks Built Before 1-26-06:
Before diagnosing the instrument cluster for this issue, first check for codes stored in the ABS
module or engine computer. If there are no relevant codes, check the vehicle speed input to the cluster while stopped. If the input and the displayed speed do not match, drive the vehicle and monitor the speedometer at 0, 20, 40 and 60 MPH as indicated on the scan tool. If the speedometer is off by more than 5 MPH, replace the cluster. If the speedometer display matches the sensor input, diagnose the ABS
rear axle speed sensor and wiring, and the ABS
module wiring. For tachometer concerns, check the RPM signal in the engine computer against the tach reading at 1000 and 2000 RPM. If the display is more than 250 RPM off, replace the cluster. Broadcast Message 5165, TSB
In your case, start w/ the rear speed sensor.