It's not a breather valve. There is no valve in an XJ 4.0. As your Autozone person points out, it's the fresh air inlet for the crankcase vent. The little hose at the rear is where the vapor is supposed to be sucked back into the engine.
As long as you have a functional hose and a grommet that fits, it doesn't have to be exactly the same as the factory original. If necessary, for example, you can make a hose sleeve for the broken part at the junction block. If you can't find that one-piece elbow, you can even find another elbow and put it through a PVC grommet that fits the hole in the valve cover, as long as it's tight, and has a bore similar to the original. It does need to be there, though, for the engine to breathe right and not pollute or simply to spew oil all over, so fix it asap. If Autozone can fix you up with something that fits right, do it.
