I managed to get a joint splitter and that did the trick -- just barely -- it was really stuck. I thought the joint splitter would break first.
A new, frustrating complication though: After getting the new TRE on the other end and attempting to screw on the nylon nut, I've discovered that the TRE threaded bolt is moving with the nut and not allowing me to tighten it. This is my first TRE replacement and I'm wondering if I have a faulty though new TRE -- it's a Lemford. Is there a trick to get the bolt to stop spinning? For such a simple job, this has turned into a major pain in the a**.