Zero Turn Mowers

My RZ4623 ZT Mower drives fine, but when I engage the blades, they work for a while then at some point the mower begins to shake and vibrate badly. Seems to be coming from down in the deck area. I have to disengage the blades in order to stop the vibration/shaking. Was told by a friend that it may be a bent blade, but I removed both blades and correctly installed new ones. Still shakes/vibrates at some point after engaging the blades. Any ideas? Steve M