Swedd is right about the level of competition in the MJ era as well as the hand check rules. Lebron is obviously a physical specimen unlike any other seen in the NBA and hand checking would only do so much but it would slow him down a tad...and guys like Dream, Ewing, and hell even Laimbeer and Rodman patrolling the lane, I can't see LBJ being as effective in the post as he is in this era (but thank god he has finally woke up and realized what an advantage he has in the post instead of on the perimeter).
Ulitmately, my thoughts are:
MJ would dominate in this era as in any era.
LBJ would be sensational in the MJ era seeing maybe a slight dip in points, same number of assists, but I see probably a couple less boards a game or more since boxing out was the norm back in the day, plus, the size of players and physical play.