Had something similar on one of my trucks, a tapered coil that bolted to the frame. When extra weight was added it would then contact the axle. I'll look in my parts pile to see if they are still around, pretty sure these were an aftermarket item.

It's easier to get forgiveness than permission!
1946 1/2-Ton Chevy
1953 Chevy 3/4-ton Factory Stakebed