If it were me, I would take LJohnson with the third pick, or Jackson if he falls. I think there's no way the Chefs let LJ sit, especially after watching Michael Bennett in all his glory last night. Larry Johnson will get his carries, lots of them, and will turn in an outstanding season. Not to mention getting a lot of TDs. Remember how Edwards used Curtis Martin? Definitely likes using a good back heavily.
I don't think you can go wrong with any of the backs you are worrying yourself about. I think Alexander will have a strong year, too - as the Seahawks should be very good again. And Gore should be ready for Week One, and with another year of seasoning for those kids and hopefully a healthy year for the Tight End physical freak, he should be a top 5 back no problem.
Keep in mind the Chiefs are my favorite team - I should add that in. But I would consider Johnson with the number 2 pick if he signs in the next week. I think he will have a great year as one of the few options that team has. Maybe not as good as last year, but top 3 in my opinion. Jackson is really being rated highly for someone who has never entered the echelons of LT, LJ, and Alexander - yet. A tremendous talent that may be the best of them, no doubt.