normally the caps with the rubber plug are hiding a zerk. If so you can pump a little in there. you don't want to over fill either because the excess will come out the back and potentially get in the brakes.
if it's been a while I might take them apart, check for water, leaking seal and then clean and repack the bearings. Not a hard job.
https://www.youtube.com/watch?time_c...ature=emb_logo
https://www.youtube.com/watch?time_c...ature=emb_logo
these just happened to be the first couple of videos I found, not necessarily the same axle