Also I dealt with a lowered truck that did the same thing and when I asked the last owner that did the work under the truck, he informed me he just put the blocks on and then heated each set of leafs till it dropped to the desired height ...

What was happening was, you could push the back down and it just kept springing up and down like a carnival ride well... when it stopped bottoming out it bounced . just ordered new springs and shocks yesterday.

Maybe someone heated yours up to "help" the drop.
just something else to think about .

T L