Tow Vehicle Rear End Sag

We recently upgraded our tow vehicle, so now the hitch and ball sit much higher than the coupler on the tongue of the trailer (when its level). After hooking it up and putting all the trailer weight on the tow vehicle, and not the jack, the rear end sags. We have a weight distribution hitch, but that doesn't seem to help. What could cause this, and how do we fix it?
Thanks-