Likely some type of canker. From your pictures looks like it is infected in a number of spots and probably isnt worth trying to save.

I just came inside from the tree. I decided to cut out the damage and see if I could save the tree before just trashing it. I found a borer. Now I'm not sure if it is best to get rid of it or still try to save it.

What part CA are you in? I always thought of borers as being a problem in other states, thats too bad.

The central valley. I googled to find out what it was and from what I saw they are everywhere but drought stressed trees are more susceptible. California would be great for them right now if that's true! Do you have any experience with them? I hate to lose the tree but I think I might have to take it out to keep any new trees safe? But the neighbor behind me has a ton of trees and they are always diseased and they never treat them so I wonder if it is even worth the trouble.

I haven't seen any definite borer activity here in coastal socal. I have seen some weird grey/black moth looking things exclusively on my plum trees (in socal) that "jump" from limb to limb, they dont look like the pictures of borers i've seen, BUT i have had some sappy little holes that I've cleaned out with a needle just in case (the holes healed over and didnt ooze again).

I'm no expert, but if it was me i'd personally probably just dig the tree out and start over with a clean slate, unless the problem is on limbs you can completely cut off and see if the rest of the tree can recover or not? You might want to look into preventative sprays also for any new trees you plant