Pick-up trucks and overland 4x4 vehicles are work-horses, built (hopefully) to handle heavy loads on their axles while retaining "driveable" handling characteristics, even with uneven weight distribution. With a front or rear-heavy load, however, tire contact patches will be larger, increasing tire wear, and causing heavy steering (front-weighted) or potential under-steer (rear-weighted) at higher speeds. The accompanying loss of suspension/ wheel travel from "squatting" your rear with a heavy load is a common problem for 4x4 SUV and truck owners with heavy trunk loads or truck camper units, one that requires a solution when traveling long distances on rough terrain where deep suspension articulation is likely. There are several approaches to this issue, and like everything in the overlanding world, the solution depends on your specific vehicle and intended use.
For leaf-sprung trucks seeing mainly on-road and light off-road duty, adding a suspension leaf or heavier leaf pack might do the trick, but if loads are uneven, such as the case in many truck campers, then the system must be able to work at each individual corner. A set of independently valved air-bags, such as Firestone's Ride Rite system, can help level uneven bed loads and keep your vehicle stance in line. Airing down the bags to just a few PSI will allow for some suspension articulation while retaining the dampening effect of a "bump stop." Some airbags, including the Ride Rite, can also be top-mounted-only with a polymer bottom cradle, allowing even more articulation and reducing the likelihood of an airbag tearing under deep suspension flex. For more strenuous off-road articulation, and front loading for use with implements such as a plow, heavy winch bumper, etc, a suspension enhancement system such as the Timbren front or rear axle SES system, or the Firestone Work-Rite or Level-Rite system, is a simple, durable solution to excessive suspension sag and sway, as well as providing a beefy bump stop-style cushion against road shocks and body roll. *Both the airbag and bump-stop style systems are made for each specific vehicle and suspension travel, so be sure to discuss your application with Pete or Matt Henwood from MLO, or your local dealer before ordering a kit.
See below for video links to these systems in real world applications: