Engine vibration

So I have friend who had a similar issue. Bad vibration around 3K and up. Thing would rattle so bad it would shake your teeth. Put a new drive shaft on it checked all the angles did all kinds of stuff. About a year later I was helping him put a different torque converter in the car and I noticed that it only had one dowel pin in the block. Put the second one in and now the car is smooth as glass.