Replaced the rear sensor. Cranked it up. Gauge read "high" long before the scangauge made it to 180.

At scangauge 183 the thermostat opened right on time.

Tried another entire dash gauge cluster. Temp gauge almost immediately went up to "high" and stayed there.

It is reasonable to assume that the Scangauge, cluster, and sensor are good.

I allowed the vehicle to run with the radiator cap off and the heater on for a good 20 minutes at least. It was on jackstands with the nose elevated. There were no bubbles coming from the radiator. Top temp on the scangauge was 188 if memory serves.

Any ideas appreciated. Thanks!