both locks for both doors on my 99 ranger have stopped accepting my keys. It still works in the ignition though. Can I change the locks out? How hard is it and where would I find replacements? Ebay?

 

Most parts stores will have door lock sets, there is a Left and Right to these but most can be reversed as needed by removing and reversing the plastic bar holder

Not hard to do on most years
Open the door and look at door edge near lock
You will see a metal tab
If you pull it out you can then pull out the lock cylinder, carefully
It will have the lock bar attached
Remove bar and put it on new lock then reinstall

If bar comes off you will need to pull door panel, worst case, so not that bad, lol

A lock smith can "rekey" most new cylinders to match ignition key before you install them

You can also buy complete lock sets that includes ignition key cylinder
But if you have PATS anti-theft system that can get expensive, have to have new keys added to the system
But if you currently have 2 working PATS keys you can add new Keys yourself BEFORE swapping out, you will need a non-PATS key cut for current ignition to do this

 

Any recommendations for maintaining/servicing locks?

 

Keep them oiled.
I like Tri-Flow, the Teflon keeps things working smoothly.

 

Hmm, it has occurred to me that for some reason, i don't have a dry PTFE lube.... i'll get some ordered.