Correct Idle Speed?

I have a 96 silverado 5.7. I was wondering what the correct idle speed should be. The reason I ask is because when it is in park it seems to idle just fine. When I'm in drive it seems like it is maybe idling too low. For instance when I'm sitting at a red light I feel a slight shudder almost like it has a sporadic misfire. If I put it in park it goes away. Any insight to what this may be? Thanks