That threaded tube is a specially sized tube basically meant to be a metered orifice for the crankcase ventilatin system. If it is too large it will cause your idle to redline due to essentially causing a vacuum leak at the intake manifold. Probably better off buying a used valve cover off ebay that happens to have the tube still on it.
Note: everything I just said is 2.5L specific. The 4.0 has a similar condition but it's metered orifice is built into the second plastic elbow, at the rear of the valve cover. The 2.5L has the metered orifice as the threaded tube at the front of the VC, and a free flowing CCV elbow towards the center or rear of the VC. The 4.0 has 2 plastic CCV elbows, one of them metered, which is at the back. Helps to provide these details so that people don't read the conditions for one motor and apply to the other, since they are basically equal but opposite in how they work.