1999 Vortec V6 with vibration issues?

I have a 4.3L v6 from a 1999 GMC in my Toyota pickup. I slipped into a deep spot in crossing a stream and the engine hydro locked. I emptied the cylinders of water and started the engine a day later. It fired up and as it settles close to at 400-500 rpms, the engine shakes very badly. Not a simple vibration these engines are known for, but enough to shake the whole truck. Give it gas to about 600-700 rpms and the vibration is gone. Anything above that and the engine runs fine. Any ideas on what might have been damaged? I know engines are usually severely damaged by hydro locking, but I'm curious since this engine will start and run fine so long as it isn't left to idle so low.