my95, some more details for you:
Does a failing joint cause the boot to leak or crack ?
Generally it is the other way around: the boot starts to leak and as the joint loses its lubricating grease, it wears out.
If the boot is leaking, does it mean the joint is also failing and should be replaced ?
Not necessarily. If it is a slow leak, like a pinhole, there still could be plenty of grease and the joint could still be in good shape.
Is it advisable to replace the joint(s) at this time: car has 95K miles ... or just wait until each one starts to go ??
I don't think you should replace them until necessary. Just keep an eye on the boots like you are doing, and when the boots are being replaced it is easy to inspect the joint itself.
what should I expect in $$ for having a Ford/Mercury dealer fix it ??
The parts shouldn't cost much. I paid ~$60 for a kit which included the boot and the CV joint. And I'm guessing 1 - 3 hours labor. Good luck!