03 F150 - Shuddering when accelerating

I have an 03 F-150 ( 5.4 engine) with around 200,000 miles. I have not had the truck long so I do not know much of its history. The truck runs very well but with one exception. When I am at cruising speed the truck runs smooth but when i depress the accelerator a little it starts to shudder and feels like I am running over something. When I depress the accelerator a little bit more the shuddering stops. Example: Cruising at 55 (running smooth) depress the accelerator a little ( shuddering), depress accelerator a little more the shuddering stops. I cannot determine it this is a problem with the engine or with the transmission. Any help would be appreciated.