Wow, 40K and you've never repacked the front bearings? Ouch, grease is way cheaper than new spindles and bearings if you wait until you hear it. I'd rather do preventative maintenance instead of more expensive repairs. Even if there is sufficient grease in there, the will need to be tightened due to wear. And it is not uncommon to find water in there is you have made in deep crossings.
The backs ARE sealed and do not need to be services as they are lubricated by the diff fluid.