While draining my radiator I noticed a grease and crud build-up on my front driver's shaft. I'll accept the boot needs replacing, but are there any signs to determine whether the entire axle shaft needs replacing? AND if I replace one axle shaft and boot, should I replace both for reasons someone is willing to explain? I've spent a lot of money this winter replacing parts that turned out to be good as new because I was told it's best to do so before I started wrenching.
99' V6 5spd w/ 215K on it

Not the best photo I know.
99' V6 5spd w/ 215K on it

Not the best photo I know.