By all means, look at the attaching points, but also inspect the springs themselves.
Look carefully at the "good" spring on one side, then carefully compare it with the other spring, in detail. I think you'll find that a leaf or leaves is bent, broken, or missing. May be broken and the broken end missing.
Do you have an E-350, or an E-Super Duty (which became known as E-450 a few years later)? Now, they're called E-350 Super Duty and E-450 Super Duty. If your Ford door jamb sticker says the GVWR is 14,050 pounds, then it's E-Super Duty.
Anyhow, Leaf Springs are simple and look massive, but they still don't last forever. Spring shops can repair or replace your leaf spring packs. We had an older Class C on Ford E-350 and added a "repair leaf" to each side. A shop can confirm this, but I don't think I'd put a repair leaf or leaves in one side without doing the same to the other. To put it another way, if the third leaf down from the top is broken on the right, I'd pull the corresponding third leaf down on the left spring (even if apparently OK) and put the same repair leaves in both packs.