Another image.
This picture shows the ambulance sitting fine.
FWIW
I was wondering why it sat so low too, but it probably had 5 or 6 Secret service guys and their weapons riding in back.
The way I understand it, the coffin they hastily procured in Dallas was extremely heavy. A White House staffer told them to “go out and get the best they could buy”. They got the most ornate one they could find which had a lot of brass and metal on it.
And yes..the Secret Service guys also added weight.
JFK was already dead at that point, so little need for Secret Service protection, plus that is a military hearse, so the body is being moved on a military base.