If this is the scene I am thinking of - just before Pickett's Charge - I don't think it is a white mini van. I noticed this the first couple of times I watched the movie. Upon closer inspection, it is a white battle flag being carried in the background.
If this isn't the same scene. Great find!
It's a battle flag, OK? It's not a minivan. If you watch early in the Pickett's Charge scene, when the Confederate lines are forming and during the march to Cemetary Ridge, you will see a rider carrying this flag in at least two separate shots.