Grease fittings

Did you get the broken piece out? Are the threads viable? If the zerk is in the u-joint, can you replace the u-joint? I can't see anything from here. You might use a tap and die set, try a tap/ threadchaser, see if it threads in ok. Hopefully the tap has the size stamped on it. You might call your local dealership, talk to the service department, see if they can give the proper size?

I think there are two common sizes of zerks and you would likely have the smaller size and it would obviously be metric. An easy out may take the broken bit out and there is already a hole in the center which will make it easier.