The axle tubes are basically spot welded through a hole on the carrier casting. You can see this weld if you look at the carrier, from the back of the truck, at the left and right of the differential cover. Welding the tubes to the carrier will strengthen bond between to two and help prevent carrier to axle slipping under high torque situations; wheel hop being one of them.
I welded mine in short/opposite sections and switching sides to keep the material from getting to hot.