If you drive a lifted truck you may find that the rear sags more that it did before you installed your lift kit. This is particularly noticeable if the lift kit you installed uses replacement rear leaf springs. They are generally designed to provide the lift with a higher arched leaf spring but to retain ride quality they reduce the spring rate so that they sag more under load. If your truck was lifted in the back with blocks then the amount of rear sag may just be more noticeable because a lift kit generally doesn't lift the rear as much as the front so any sag looks worse than it actually is.
What is the solution to sag in the rear of a lifted truck? Add a set of AirLift rear air bags. But the bags are designed for trucks with a factory ride height so you have only created a new problem. But AirLift comes to the rescue with their spacers designed to go between the air bag and the mount thereby putting the airbag in a position where it is designed to travel at.
Even if you have an air bag kit by another manufacturer the AirLift air bag spacers are designed to work with most brands od air bag kits.