If they are Dexter Nev-R Lube then they cannot be lubed. They do fail, I have them and had one die near SLC. You can look on the Dexter site and find a manual for them. All you can do is check the wobble or runout of the bearings to see if they are about to fail.
The wise Nev-r Lube owner at a minimum carries a set of rebuild parts or better yet an extra hub with bearings already pressed in. Finding someone on the road to press in new bearings can be trying, took me three days and they bungled the job.
They cannot be replaced with other bearings unless you replace the whole axle.