This is a complicated sounding system, but it really isnt when its all broken down. Basically the vehicle speed is determined in a linear fashion. Each wheel speed sensor sends a signal to the ABS computer. The ABS computer creates a single signal based on the 4 wheel speed sensor inputs and sends this signal to the instrument cluster. The instrument cluster does 2 things. First, it takes the signal from the ABS computer and makes the speedometer work based on that signal. Then, it sends another speed signal to the engine computer. So, basically,
Wheel speed --->ABS computer --->Instrument cluster(speedometer)--->Engine computer
Since your speedometer is working, you can determine that everything is working up to that point. So, there are 3 things that could be happening here.
1) The instrument cluster is not sending an output signal to the engine computer
2) There is a wiring problem between the instrument cluster and the engine computer
3) The engine computer is not registering the signal from the instrument cluster
I would look very closely at the wiring harness on the inside fender wells to see if there could be any damage. Rodent damage is very likely and can happen due to squirrels or pack rats in almost any environment.
If you see nothing there, we will need to do some more in depth diagnosis to isolate the cause of this code. It will require having a good digital volt meter, some backprobe pins, and knowledge of auromotive 12 volt circuitry.
This code CAN cause the shifting of the transmission to be erratic, but if there are some major shifting issues, it could be caused by something else.