>>Why do you have to replace the whole axle?
You don't, of course. For me, the biggest cost of any job is time and effort. Parts are typically less than half of the total. I prefer to put in a complete assembly and start new, vs. waiting for the next component to fail and doing the job again.
>>If it is only a tear in the boot can that be repaired and more grease introduced?
Sure. Unfortunately, the probability that zero dirt has migrated into the boot and contaminated the business side of the CV is pretty low. People bet both ways on things like this.
>>Also, how much better is the OEM part? This part on RockAuto is $45.
No idea. I usually wonder how much worse is the cheap part, and how much do I care if it fails early. If its easy to replace and doesn't impact safety or driveability, buying the cheaper part often makes sense to me, too.
>>Can I do this and save a couple hundred $$ or am I missing something?
No doubt you can do it if you have this forum, YouTube, patience and the tools.
>>If these operate independently of each other what is the advantage of replacing the other one if it isn't broken?
Its a philosophical thing. With pairs of equivalent parts, one will fail first. If they see the same conditions, they can be expected to have similar lifespans. If I plan to keep a car, I assume that the other one will fail soon and replace both parts as a preventative measure, unless its obvious that the failure is not just normal wear and tear.
As with so many things related to cars, your mileage may vary.