You really need to test them, and just inspect the rubbers closely. If they are completely tight and the rubbers are good, then maybe you're ok. If there is even a little play, or if the rubber has even pinholes in those cracks (or the crack going most of the way through), then I say just replace them. My pastor's 4runner had no play, but a cut in the rubber, and it ended up failing I think about 6 months later. IMO, this is really a 90k lifetime part, so I'd replace it after about 100k no matter what. That's not a very big cost when spread out over so many years, likely 5yrs at minimum.