Axle truss, it's to help keep the axle from folding up when you the truck is sent off a jump and it comes back down to earth.
Most of the time they were used on the from axle as that is where all the weight is and because of the weight fold easier.
BTW most of the time they did not work.
Dave ----

 

Ya an axle truss... 1980's 4x4 bling that didn't actually limit deflection or perform a useful function. Usually was installed with multiple shocks, also for looks.

It does however limit ground clearance, and could get hung up on rocks to help you get stuck.


Now days functional axle trusses are made out of laser cut plate steel and welded to the tubes and housing.

 

I don't necessarily disagree on their effectiveness (or lack thereof) but don't think they were simply bling either.
Of course in the '80's they were used mostly as a counterpoint to the neon pink and green shock boots and bushings(!) but trusses like this were actually used by the low-buck desert racers in the early days, and by those not wanting to bust up another axle tube. Like FF said, usually on the front so the long side didn't taco on the smaller axles like Dana 30's under Broncos and such. They may not be pretty, or even that strong, but I bet they did save a front axle or three over the years.

And guys that had seriously overloaded rear ends did reduce flex and bearing/shaft wear over time.
But yeah, most were just looking at the cool pics in the cool kid's catalogs and were never going to tax their rigs off-road enough to justify a truss. For those, they were bling. But for some of us, that actually used their rigs and didn't have enough money for the big shovel type, or a custom fabbed unit, they were what could be got.

Paul

 

"What’s this bar across my rear?"

It's for protection! They were put there to keep you safe from Aliens looking for Uranus!