That's a tough one to diagnose without driving the van. The only thing that comes to mind is to make sure your brakes are not dragging. If they are, you'd notice a burning smell after driving a bit. Or you could jack up the car and make sure each wheel turns freely. Also, if you have traction control, try pushing the button to disable it to see if that makes a difference. And, of course, if you haven't done so you should see if there are any error codes. Parts stores like Autozone will read them for free. Let us know how it turns out.