I think something that tends to go a little unnoticed in the whole run game discussion is the pretty obvious tendancies the packers have with their running backs. You can usually see from a mile away, which type of play(run or pass) they are going to run by who is in the game. If grant is in, chances are very good that it will be a run, he isn't really much of a threat in the passing game, so teams load up to stop the run. If you've got jackson in there, chances are very good that it is a passing situation, and seeing how wynn has basically been filling jackson's role when he's been in, same story.
Keeping all of this in mind, i think its simple to see why Jackson/Wynn have had success when they've been in. Now of course this sort of logic is not always the case, but i think if you went back and looked through a lot of the film, it would tend to be the norm. More than anything this suggests that the packers need to be more creative with their personnel, and more willing to try Jackson/Wynn on 1st and 2nd down, and just plain try them when Grant isnt having much success. I think the unwillingness to give either of the other two backs a shot when Grant is being continually stuffed was one of the greater downfalls of the 08 packers.
Really I think all 3 of them are quality players, and all of them have shown it at one time or another, but if you look at a team like the Giants, who have great success using 2 or even three backs, you see them switch it up, and really go with what is working "now" instead of just trying to force feed "their guy" just because he is "their guy", even when it isn't working.