I can feel a vibration when im in park and throttle up to 1300 rpm nothing before or after just at that rpm. What could this be?

Perhaps something in your exhaust is loose and resonating a certain harmonic from the engine.

Its a slow rocking.

I am guessing but injector ballance is turned off above idle so any imballance of dirty injectors comes into play. My 3 trucks and 2 friends trucks have a very slight lope/roll off idle , its not noticable when your driving . Try some injector cleaner for a 100 miles and see if it get any better.

Take it to the dealer. There's a reflash available for the lopey idle, smoking, reduced mileage- mine was doing the same thing. Does it get worse after you drive on the highway?

The TSB no. is 06-06-04-066A

No just when its in park and i throttle it up 10 1300 rpm and hold it there. after 1300 rpm its runs smooth and before 1300 rpm. I asked the gm tech about the reflash and he said he hasent heard of a such thing.