Someone stated that if Nissan put a plug there, then it must not be serviceable.

Wrong!

Nissan put plugs in the old WD22 Pathfinders and Pickups in all the tie rods, upper and lower ball joints, center link and idler arm. It was a cost saving measure, that the Dealership was supposed to upsell during future maintenances.

Don't ignore something just because there's a plug there. If you offroad your rig, service it. How many X's actually make it offroad? Nissan was just playing the check book and didn't put 'em in in some of 'em.

And replacing the cone zerk with a regular one, isn't enough to throw your driveline out of balance. You're not supposed to be spinning it up fast enough to see that anyway.