Regardless of the level of play from D-I through juco, NAIA or D-III; I think you'll find that unless you're an "invited" walk-on you face extremely long odds of making a team. Many schools invite players to walk on because they don't have scholarship money to give them, but want the player on the team. If you're one of the other type, the type that just shows up hoping to earn a roster spot, you face very tough odds, and usually have no chance of making it.