engine shaking with no codes. help!!!

my 2004 lb7 duramax with 144k has over the past 6months developed a vibration or shake at idle the tack stays at a constant 680rpms. i replaced the fuel filter about 2k ago and the injectors where done about 14k ago. im at a lose because i have no codes and it seams as if there is a miss because the engine is shaking back and forth and advice or insight would be deeply appreciated. hell if some one points me in the right direction i would be willing to through them a little cash there way