ok, just so you know, from time to time i'll shift through my gears ( i have an auto tranny) i dont shirft that aggressively, just gives me something to do and i can accelerate much better than leaving it in Drive. anywho, today i had to slam on my brakes for some moron who thought that because they drive a mercedes, they get the right of way no matter what. when i started to accelerate i had it in first and shifted at 4k. right when i put it in 2nd (3spd) i felt a vibration for about a second or so... never really noticed this before so it got me curious. at the next stop sign i started in 1st and shifted again at 4k...same thing. then i shifted into 3rd at 3.5k and had less of a vibration. what is this?? is it serious??

It's called shuddering and your transmission doesn't like it. Let it shift on its own, it's automatic for a reason.

2002 Cavalier 2200 5spd