I cant say that I know exactly what you are talking about with everything, but I concur that your voltage should read right around 5V at full throttle. This is the "max" for any inputted voltage for the PCM for any sensor, including MAF and temp and everything! so since you arent getting the max voltage for throttle, its not inputting the max amount of fuel for full throttle. I am also betting that your truck is thinking that you are really weird for never using full throttle!
To fix it it could be a number of things that might need to be replaced. First I would check your actual sensor to see if it is outputting the right voltage, and if its not then replace it (bet you saw that advice coming
). If it is correct then I would try and check the wire to make sure its connected at both ends (might be tough to do, but a loose connection could be a the problem. If that all looks good you might want to see if you can get the computer to see 5V and to display that to you, either feed 5V into the wire, or see if the pedal works with the truck turned off.
Hope that helped, but just a precautionary for ya, I am not a mechanic, I just told you what I would do if I saw the same thing
Merry Christmas!