I have a 2003 1500 with the 4.7 and when trying to start my truck after its been sittin for a few hours, it will start to turn over but it'll grind and stop. Ill leave the key turned to the starting position and hold it there and after 15-20 seconds it will start. I replaced the starter and the battery and had the alternator checked.

 

I wonder if you have some water in a cylinder.

 

You could have stripped out a section on the flywheel, when this happens the starter will turn the engine fine until it hits that stripped out spot and that's when you hear the grinding. If you think that's the problem pull off the starter and look at the flywheel, you might have to turn the engine over to check the whole flywheel.