Why does my truck do this?

I have a ford f150 xlt 2004 v8 5.4l and i've noticed that when I take my foot off the break it accelerates but like if id be stepping on the gas. I know that by taking my foot off the break itll mve but what i'm trying say is that. Like when you have it on park and you step on the gas well it does that but when I put on drive it tends to go on its own. Why does it do that

Well your truck could have so much power your truck just wants to pull when there's no gas. Most cars do that my sisters Pontiac grand am v6 would just pull without gas. I don't know how much of a pull your talking about though but not to much to worry. Just means your trucks a powerful truck

Check for a vacuum leak. Somewhere between throttle body and head. Possibly vacuum hose and check heater/AC control vacuum motors.
With fuel injection the O2 senses lean condition and ECU adds fuel to adjust. This causes engine to run faster.