$10 spent on a CAT scale weight at a truck stop can help with your troubleshooting. The scale provides weights for Steer Axle, Drive Axle (of a semi tractor) and Trailer it's towing. Go inside (sometimes to what's called the Fuel Desk) and ask how to position such a short vehicle as a minivan that it'll capture the Front and Rear Axles individually.
Could be ALL your springs are shot and the front is sagging more than the rear. An alignment shop should check "Ride Height" before starting work. That says there's a spec somewhere that you can get and check.