My mistake, by speedo I thought you meant your whole cluster, not the individual gauge.
If you want to confirm your cluster function you could've used the built in diagnostic procedure(actuator test) without removing it and just writing down the codes that appeared on the display.
To perform the actuator test put the key in the off position, hold the trip odometer button down, turn the key to the run position and release the trip odometer button. The cluster will begin its function test of its circuitry, gauges, and indicators. If you have codes record them down and refence the code chart. 03, 04, and AC are ABS related. 8J-12 through 14 in the FSM for 03 trucks have the code reference chart.
Additionally from the 03 FSM in regards to the speedo gauge itself
"• Communication Error - If the cluster fails to receive a speedometer message, it will hold the gauge needle at the last indication for about three seconds, or until the ignition switch is turned to the Off position, whichever occurs first. After three seconds, the gauge needle will return to the left end of the gauge scale."
Did you notice this?