Did you guys solve this problem?
I'm having the same issue. Noticed it after rotating tires, front left to back left and back to front. Had tires balanced by two different shops. Had it aligned and it got better but it still rattles on the steering wheel when accelerating above 55, 60mph. No bump felt on the tires. Are the rims directional, meaning is the back only meant to be on the back?

 

I never figured out what the problem was. I do little freeway driving with the van except on road trips so I don't encounter it alot. But when I get it up to 80 mph its quite pronounced. I have put new tires on it and I believe they have been rotated once already but I cant remember. I will be rotating them again soon to see if there is any effect, but Im not confident this will be the solution as I am more confident I have already rotated them with the previous tire set. I have aligned it several times as well to no avail.