I will give you the benefit of a doubt .. Assuming you are being truthful .. What you should do .. Is get a lawyer, turn yourself in and face the consequences for your behaviour. Please note I have not added superlatives .. Which I REALLY want to .. But will refrain because it would probably overpower the point I am trying to make.
"Cracking someone across the back of the head with a 2 X 4" is not just a matter of assault .. It could be far more serious than that. Something that serious could be elevated dependant upon WHY you did it .. If it was self defense, you might have a case for defense .. If you were the aggressor and your intent was to permanently injure this 'guy' (or worse) .. Then you may face some very serious charges...and rightly so .. Especially if he has sustained a brain injury.
In any event, I'm thinking you should should do some deep soul searching and realize just how serious THIS is .. Perhaps some anger management might also be warranted.
It's not only concerning because of the damage YOU have caused, but because of your out of control intent to injure (which just as easily could have killed the guy) AND because of your choice to handle your differences in such a violent way.
Are you really prepared to go through life settling your differences by being so destructive? Think about it .. and find your resolve, because if it is ... also be prepared for all that accompanies that level of violence.