Hi Mark, I would bet that the delrin Z nut would wear out before your coupler. The coupler is machined aluminum, whereas the Z nut is delrin plastic. Also, the threaded rod is constantly rubbing against the Z nut, threading in/out in order to raise/lower the Z axis. Meanwhile, the coupler holds the motor arm and lead screw stationary. So, it is less likely to wear.
First step. I suggest replacing the Z nut. But, if you're up for it, you may want to go ahead and replace the coupler and threaded rod as well. The parts are inexpensive and you may avoid having to re-diagnose if you replace all of those Z axis parts at once. See links for replacement parts below. I hope that is helpful.