They weld aluminum all the time, it can be done. BUT it's very hard to properly weld an aluminum housing like this one. You need to preheat the part to be welded in order to reduce residual stress in the metal. Also, aluminum burns, so the alloy at the site of the weld will be of inconsistent quality unless you really know what you are doing, what alloy it was made of to begin with, etc. Depending on where on the tranny the patch is, it could be subjected to internal stresses when it is doing it's job of propelling the truck forward. Welding will produce metal particles that will end up inside the tranny, in the oil and in between the mechanical parts, unless you disassemble the tranny beforehand, which would be good if you wanted to preheat it properly. So I think it can be done properly, but it could end up costing a lot. Or you could take the redneck approach and weld it in place, and cross your fingers.
2003 SE-7 Disco II, 132,000 km.