we sure could use a lot more information here too,,, like a bit of description of "speed up by itself" (does it go wide open throttle? and stay there? is it a surge when first starting up? is it surging at wide open throttle?)
Make, model, and serial number would be a big help too, as would if it has factory or aftermarket carburetor.

remove the plate on the side of the governor and check the spring in there, if worn out it can allow the engine to surge.

I think you might have a problem with the throttle assembly under the carburetor. I have had them seize up, also the throttle cable can be a problem. If it's electronic throttle you need the set it by PC

if it speeds up and slows down while youre driving then you need to adjust the governor
if its doing it at idle the governor might be broken or something could be wrong with the carb like ed said
but you need to give more info

model, year or serial number of the truck might help since it is much more likely to be a carburetor problem than internal to the engine.
my "instant bet, without better info...", the little screw that holds the return spring on the Carb throttle shaft has come loose.