When was the last time the bearings were cleaned, inspected and repacked? I know Dexter recommends repacking every 12 months or 12K miles - I do that at the beginning of each season along with brake inspection and adjustment.
If you are unsure of the type and quality of the grease then probably best to redo them with a quality grease that meets the axle manufacturers recommendations. I wouldn't just pump more grease in at this point, especially if you don't know what kind of grease is currently in use. It's not recommended to mix grease types to avoid compatibility issues. Also, when you have things apart, if there is evidence that grease was previously pumped in I would recommend pumping some new grease in while they are apart to flush out any of the old grease that would still be in the supply line between the zirc and the back of the spindle. That way if you decide to use the zircs later on you won't contaminate your new grease.
TT: 2015 Jay Flight SLX 195RB Baja Edition
TV: 2014 RAM 1500 Big Horn CC, 5.7L Hemi, 8-spd, 3.21LSD, Factory Air Ride/Tow Pkg, Andersen WD/SC