I'd try a "known good" guage first to see if your old guage is reading properly before I started digging into stuff.
A thermostat is cheap and easy to replace. If you replace the thermostat, the one that comes out isn't neccesarily the temp you want to use. Go with what the factory recommended for your truck. Some people put a higher temp thermostat in thinking that the heater will throw more heat. They most likely have a plugged heater core. If the heater core is plugged, it is probable that the radiator is somewhat plugged too.
A water pump will usually start leaking before it goes altogether.
I saw a radiator that was plugged up on the "outside" with mud and bugs. It ran for years but overheated when under heavy load.
