I didn't answer on the other thread because there's just not enough information, we're having to guess.
How did you process the birds? Were they calm or did you have to chase them around ? [Anecdotally, this seems to be a concern for beef and hog processing. I'm not *sure* it matters for chicken, but I try to keep them as calm as possible. If I scare one or it gets away from me and goes running around, I put it back in the pen to do later.]
What kind/breed were they? What hatchery or store did they come from? (If only so we can avoid it!

)
Now... exactly how did you handle it and cook it? Sounds like the first one was cooked immediately, which can work *if* it goes in the pot right away. If you let it sit around and begin rigor mortis, then you need to wait that out.
But this one rested for three days, right? Or... actually it sounds like it rested one day, then you brined it for two days. (That sounds like a long time in the brine to me, but I'd expect it to get mushy and salty, not tough.)
Here's my suggestion:
Do you have a slow cooker (crock pot)? Put the next one in the slow cooker with one and a half cups of your favorite salsa (I use Pace medium picante sauce). One hour on high to get it started, then three to four hours on low. I *guarantee* it will be falling off the bone. Take it out, debone it, shred the meat and put it back in the sauce. Add 1 teaspoon cumin and the juice from one lime. Stir that around and heat it for another 15 minutes. Serve over rice, or use in soft tacos, or as the filling in
these baked taquitos.
-Wendy