a problem with an '89 idling engine

[yourdeathwish]

My truck overall runs pretty well overall, not a whole lot of problems and plenty of power, but i have this problem with this constant vibrating on the engine. When i drive it enough to warm it up, it starts to idle when im at a stop, but when i first start it and its still cold, it doesnt do it. Another thing ive noticed is that when im stopped, and put it in nuetral, it doesnt idle anywhere near as much. can some tell me what might be the problem and how to fix it? I think it might have to do the the timing...