They are just plug welds...it wouldn't matter one bit if the weld held to the cast or not. All that is happening is that the casting is preventing the "plugs from turning"
For instance...say the hole is 3/4" in diameter and before welding you dropped in a piece of 3/4" coper tube in the hole. You then weld it and fill the hole right to the top. Your weld will be fully welded to the steel axle tube but nothing else...This would probably be enough to keep the tubes from ever turning on a standard axle or pulling out of the housing but it wouldn't take much to scheer off the welds and turn the housing on the tubes with the torque we create.
I have no pics yet for you guys but on the Dana80 in the front of the new truck my brother and I are building those plug welds only come up off the tubes into the casting by about 1/4" or a little more. It was orginally from a Ford F350 Cube Van and had the heavy 3/4" axle tubes (the dodge uses tubes that are thinner) and some of the 10-bolt Dana80's in the fords had 1" thick Tubes.
RyanB