By design, your rig's axlehousing can withstand a significant amount of abuse. Its design parameters are exceeded, however, when you bolt on larger tires and engage in aggressive 'wheeling. This is because the addition of larger tires creates more leverage on the axletube, and on its attachment points to the diff pumpkin, than they were designed for. This is enhanced further by the additional weight of those larger tires, and by the addition of positive-offset wheels, which move the heavy tire-and-wheel combination even further out on the tubes, creating even more leverage. Clearly, axletube length plays a part in this equation as well. The longer the axletube, the more leverage is produced.
Frequently, the ultimate result of this leverage is that the tubes can bend. When they bend, the tubes are no longer aligned, and this changes critical measurements on the components inside the housing. Often, this results in premature failure of the bearings, broken axles and even the complete destruction of the ring-and-pinion. A telltale sign of a bent housing can be wear on the inside of your rig's rear tires. Dana 35C housings are notorious for bending the axletubes up and back over time when used with 33-inch-diameter or larger tires, and this cocks the axleshaft in the differential, which can cause havoc with the internals. We're told that this is especially a problem for those weak 35C axles fitted with lockers, because lockers are machined to very exacting tolerances and can't endure axle misalignment.
You can easily exceed the design parameters of your axlehousing even without larger tires and positive-offset wheels. A good example of this would be by catching air with your truck. Let's face it; trucks just weren't designed to fly. Sure, we'll be the first to agree that big air looks great in photos, but the damage that results can be staggering. It's a variation of the old joke, "It's not the fall that kills you, it's the sudden stop at the end of it." An illustration of this is to use this formula: force = mass x acceleration. If your new Super Duty weighs 5,660 pounds and you launch it off a jump onto a hard dry lake bed at 60 mph onto the front left wheel, you'll not only get gnarly photos, but you'll also get 339,600 pounds of force on the axletube. This is not a good thing, and it may cause your axlehousing to suddenly resemble the lines on a Vermont road map. So the question is; what can you do to beef up your truck's axlehousing? The answer is to install an axle truss.
An axle truss is an external reinforcement for the axlehousing. The most familiar style of truss is the type that bolts onto the bottom of the axle, and there are many of these types available in the aftermarket. The upside to this type is that it does offer a level of support, albeit minor. But every little bit helps. Most of these trusses use straps or U-bolts to fasten the truss to the axletubes. When selecting a truss of this type, look for one that mounts as far out on the axletube as possible, because these will be the most effective in fighting vertical and horizontal movement of the tubes. The downside to trusses of this type is that they decrease ground clearance under the axle. Numerous 'wheelers will also tell you about trusses of this type getting damaged on the trail, twisting upward and puncturing brake lines. If your type of 'wheeling is of the easy variety that doesn't require attention to ground clearance, then this type will probably work for you. Many tough-truck racers don't run much suspension lift, so the center of gravity of their trucks remains low, so they have clearance issues with over-the-axle trusses. Thus, they tend to use beefy, welded-on under-the-axle trusses to help deal with the abuse of hard landings.
Over-the-axle trusses are the most popular for those who 'wheel often in challenging terrain. A major benefit of this type of truss is that they create no reduction of ground clearance under the axle. They do require space above the axle however, so you have to take into consideration the amount of upward travel that your rear axle has, and compare it with available space. If your rig has too little space above the axle, the truss may grind into the underbody of your rig at full compression. Over-the-axle trusses come in two varieties--bolt-on and weld-on.
Everyone has his own favorite type of truss, and all will help strengthen your housing to a degree. Most 'wheelers who hit the trail often agree that the most effective axle truss is a weld-on, over-the-axle truss. You can fab one of these yourself, but only if you have a thorough working knowledge of welding and fabricating. Screwing up a weld-on axle truss most likely will cause significant damage to the axletubes, resulting in expensive repairs. If you don't have the skills necessary to build your own, there are a number of aftermarket manufacturers that can build one for you, and there are several who offer beefy over-the-axle bolt-on units that you can install yourself.