If you can find a new replacement fitting, that would be the better option. It's tough to repair a cracked drive fitting and have it stay together under the stress of doing it's job.
If your cracked U-joint is made of Delrin then it won't glue unless you can find, and afford, a specialized adhesive made for Delrin.
If it's made of some other plastic, then CA/super glue, or even styrene cement might work. However because it's slippery, and strong, Delrin is often used for this kind of fitting. You can try the CA and see if it works. If the dried CA can easily be flaked off, leaving the U-joint bare, then it's made of non-stick Delrin.
Delrin can be "welded" by heating the edges of the desired joint with a soldering iron. I do this with the tops of Micro-Trains coupler boxes, which are made of Delrin. I've had several couplers come apart, including the more expensive, factory-assembled ones.
In those cases the factory welds were not done as well as they should have been.
Doing this on your U-joint would be a challenge. You would need to melt the Delrin along the length of the crack and move the soldering iron side to side across the crack to overlap a little of the Delrin plastic over the crack. Even if you managed to do that, there's no guarantee that the weld would hold together on a moving drive component. The coupler boxes aren't under the same stress.
I've not used the JB weld, that MichaelE recommends, but I like his idea of putting some sort of containing sleeve around the outside of the U-joint. I'd be inclined to use brass tubing, if I could find a size that fit, or else heat shrink tubing. However Michael seems to be a quite knowledgeable modeler, so the JB weld may work equally well.