The rear bearings are actually sealed... you can't repack them. Now if the seal blows you need to get into the dealer and have them take care of it as you'll lose all your diff fluid if it gets bad enough. They are however covered by the powertrain warranty as are the front bearings. If you remove a rear disc, which is easy, you can see if they are ok, other than that you can check for grease around the back of the disc. My parking brake wasn't working and it turned out all the innerworkings of the drum were completely greased.

Once you have repacked your front bearings on your own, it's easy to do it the second time... it's worth doing yourself if you plan on keeping with the every 15k rule.