Does the speedometer work?
Does the speedometer read correctly while driving the truck? Also, if the truck is equipped with speed control, do you know whether it works?
Okay, the most likely reason you have an OD indicator flashing at the same time that you have a non functioning speedometer is that the vehicle speed sensor signal is not getting to the Powertrain Control Module (PCM) that controls transmission shifting. The vehicle speed sensor signal first goes to the instrument cluster, then to the PCM. The error code on the odometer most likely means that you have a failure in the electronic module in the instrument cluster. Before we fault the cluster, please check fuse #8 & #18 under the dash. Inspect the vehicle speed sensor, it is located on the top of the rear axle in the center (almost straight up from where the driveshaft goes in). Inspect the two wire connector and wiring for damage. The speed sensor can be tested with an ohmmeter if you have access to one. If in doubt, the speed sensor is inexpensive and easily replaced, it has one bolt holding it in and slips straight out. If the speed sensor and wiring are okay up to the instrument cluster, then the instrument cluster is at fault and will need to be replaced (a likely situation for this error code). It can only be replaced as an assembly as a remanufactured item, or perhaps a used one. Hope this answer is not too long, but want to cover as many bases as we can before blaming an expensive part.