They are lubed by the gear oil. The only time you have to worry is if you open one of the hubs and it drains out. It will maintain the correct level on its own, but you don't want to drive around with it dry until it replenishes by splashing. Jacking the opposite side as high as you can will make oil flow into the low side.
The Constitution needs to be re-read, not re-written!
If you can't handle Dr. Seuss, how will you handle real life?
Current oil burners: MB GLK250 BlueTEC, John Deere X758
New ride: MB GLS450 - most stately
Gone but not forgotten: '87 F350 7.3, '93 C2500 6.5, '95 K2500 6.5, '06 K2500HD 6.6, '90 MB 350SDL, Kubota 7510