The guy didnt exactly say why it was rebuilt. The car vibrates in all gears and nuetral and park. It does vibrate a bit more while in gear though.

 

I just got my camaro around a month ago and sometimes experience a similar problem. It only happens in very humid conditions though - when its pouring rain and there are puddles all over the streets. I'd stop at the red light and all of a sudden feel the whole car vibrate as if the engine cycle was off beat or one of the cylinders wasn't working. Then I'd drive a bit more and it would go away. I know I do need to clean my injectors and replace my wires, so that might be my problem. I will find out soon.