truck shakes while idling but I don't know why

I have a 99 Mazda B-4000 4x4 (identical to Ford Ranger XLT). I took it in to the dealer for an oil change and immediately when I started the truck it was shaking while idling. I notified the mechanic and he came out and checked to make sure oil was filled etc.. but all I know is that is was fine before and now it shakes. What could have happened? Once I begin moving everything is normal, but it still shakes only when I am stopped and it is idling. It is a manual transmission and it doesn't seem to shake from low idle speed since when I push the gas in neutral it still shakes. This is baffling to me, does anyone have any ideas? Regards, Matt