Only way I could see Lebron in the NFL is a flexed out TE. He's too tall to be an effective in-line blocker. It would depend if he could get low - which he may be able to do with his athletic ability. His first step quickness would be tough to stop, but it comes down to how agile he is in/out of his breaks. That's asking a lot.
But realistically, I think 'no'. It's a totally different physical skill set to play in the NFL than it is to play in the NBA.