Aluminum weld yokes, for aluminum driveshaft tube. They pair only with aluminum tube — aluminum cannot be welded to a steel tube.
Check the alloy as well as the dimensions. Aluminum shafts need a welder set up for the alloy, and the finished assembly still has to be balanced.
Aluminum weld yokes exist so an aluminum tube can carry a standard u-joint series. The yoke is machined the same way as its steel equivalent, but in aluminum, so it can be welded into the tube without a dissimilar-metal joint.
Series still governs which cross fits the ears, and butt diameter still governs which tube it enters. What changes is the material pairing, which is not optional.