Weird shifting and Speedometer problems: Speed Sensor help?

[gv280z]

I'm wondering if you can help me with a problem I'm having with my truck regarding this topic: quite often now the speeds at which my truck shift's gears and the speedometer are both simultaneous wrong. Then once it decides to clear up and start acting right, usually after about 10 or so minutes of driving, the speedometer and shift points basically align themselves as if nothing ever happened. At first I thought it had something to do with my TPS 'position' on the side of the throttle body but now I'm not so sure.