RENIX has a total of three temperature sensors...
The first is in the driver's side rear of the cylinder head, and drives the panel gage.
The second is in the lower driver's side of the engine block, and reports coolant temperature to the ECM.
The third is in the driver's side tank of the radiator, and turns the fan on and off.
Since you say you don't have AC, does that mean it's been removed, or was never there? I just want to be sure.
Other things to bear in mind:
The volume tank and cap require replacing at intervals - I change mine every five years (probably too often, but why wait? The ones in good shape get kept as "shop spares" in case I need one in a hurry.) I sign and date the bottom of the tank. Replacements can be had from Morris 4x4 Center (
www.morris4x4center.com) for about $20 - both parts. That's where I get them.
Make sure you don't have a pinhole leak in anything - especially in the heater hoses. I've traced cooling problems like this down to things like that. The fact that it stays cool while you're running tells me you're getting good coolant circulation and cooling action, so there are other places you should look.
Check to see if you're losing coolant over time - that would be indicative of a leak. Isolating the leak is a matter of time and effort.
5-90