If the truck has the valve in the back end like jettas did to provide changing fluid pressures to the rear brakes to avoid rear brake lockup they can be freaky and expensive to buy new.

I suspect when you were dealing with the back brakes somehow or other reverse pressure moved a piston in that valve restoring operation. That valve has a lever if present to the chassis to tell it when the rear end is lifting up I believe. Once again your vehicle may not have this valve setup but if it does you will notice two lines going into it and two lines exiting it. Plus the lever to the chassis.