The splines inside the bottom agitator that are supposed to fit snug with the agitator shaft were worn down and couldn't catch the shaft anymore. I removed the entire agitator then pulled the top agitator part off of the lower agitator part. I then replaced the lower agitator with the new one I purchased and kept the old top agitator part (the top part was still in good condition and didn't need replacing, although I did replace the "agitator dogs" because they're very cheap and figured it wouldn't hurt). The new lower agitator part did the trick and my washer is back in action. All that I used was a socket set and 7/16 attachment. Very easy fix.
BOLT WILL NOT LOOSEN/TIGHTEN: I ran into one problem when trying to remove the agitator. I figured this info might help someone with the same problem.
The bolt connecting the agitator to the agitator shaft would not loosen and come out as easily as all the videos I watched (possibly because I still had the washer plugged in?). I had to turn the washer on (small cycle, obviously), let it fill up and then turn it off once the agitator shaft started turning. That locked the agitator shaft in place and I then was able to easily remove the bolt and agitator while the washer was still filled with water. I then took apart and reassembled the agitator (while keeping the washer filled with water and shaft locked), put it back into the water, made sure the agitator was snug with the shaft, re-bolted it back in, turned on the washer and made sure it worked, which it did. Hope this info helps.