The gauges aren't known for being accurate. Once you've exhausted the options of replacing the sending unit and making sure your grounds are good, you could consider a hack:
If you have an IR thermometer, scan for max temp right at, and around, the thermostat housing with the engine fully warmed up. You should get a reading very close to your thermostat rating. Then pull the cluster, remove the lens, and carefully pop the needle off the temp gauge. Re-install the cluster and fire the truck back up, so that the gauge's stepper motor is positioned where it *thinks* it should be.. and then place the gauge needle at the reading it *should* be at.
Does this fix the problem? Nope, but it will now read correctly when it is at the expected operating temperature. If it goes over that reading you can be pretty well assured it is running hotter than it should.
Richard