Nope you should be a little over 500rpm when warm. Even higher when cold. Is your truck stock? You should check your throttle body to see if theres any carbon build up that might be messing with the blades. Are you tuned?
Only mod I have is a home made cold air. Not tuned either. It'll do this randomly. I'll be sitting at a stop light after driving around for a while and it'll just sit there. Sometimes it'll jump back up to 500-600 then go back down. But it seems to idle fine so I don't know. Weird
Is your air filter clean? I had a MAC cold air on my GT and when it got dirty (every other month) my car would idle like it had cams in it. Sounded mean as hell but it was my air filter being nasty. Clean it up and back to idling silky smooth.