Every 600 I've seen has come stock with gear oil in it...
There are oil seals on each side of the main gear case to keep the oil in..
The outer axle housing has either a bushing or needle bearing and seal which is lubricated on installation...
If you can you should replace these outer axle housing units with the needle bearings/seal set....780091
Packing the needle bearings with a high pressure grease on installation will be sufficient...
Drilling and tapping in a zerk thru the bearing race will destroy it and drilling it "behind" the rac really won't get grease in there because the back(inner) side of the housing is open grease will just slide toward the center and not "into" th bearing...Clean and lube on installation....
The bushing on the input shaft can also be replaced with a needle bearing (780063) and seal and will be lubed by the gear oil from below so a zerk is not necessary there either...
With the needle bearing below, the upper ball bearing will be self lubed as well...