I personally just replace the axle with a rebuilt if the boots are torn because the rebuilts are such a good value. If I were to decide to replace a torn boot, I'd assess the bearing. If the lube was essentailly gone, or of there was dirt in there, I'd change tha axle as the bearing life is limited at this point, and replacement boots could be throwing good money after bad, not to mention labor. If it's still greased and clean, I'd remove all the old grease with a solvent (spray carb or brake works great) and use new grease. I'd also do all four boots.
