When the truck is pulling, the axle tries to rotate the opposite direction as the tires.
A bar from the top of the differential center section to the transmission, parallel to the driveshaft, resists this rotation without affecting wheel travel, springrate, or binding. You can hang the driveshaft hoops from it, and even make the trans end brace the Tcase.
This is used on the 4th gen F-Bodies like the Camaro.
Normally this design is accompanied by a panhard rod (side to side limiter) and 2 LCA's (lower control arms) to complete the equation. But nothing is stopping you from putting just a torque arm on.