The system is fairly simple: there's a sending unit, which is just a temp-sensitive resistor and there's the gauge, which is essentially a volt meter. This is an oversimplification, but you get the point. The sending unit is easy enough to check with an ohm meter. It's rare for a gauge to go bad.
The FSM has instructions on how to troubleshoot this problem.
Subscribers have access to the FSM, I think.
In any event, let's say that it IS the gauge that's bad. Do you really want to dump that kind of money into an older truck? I would encourage you to buy the ScanGaugeII and use it to "replace" your gauge. The ScanGaugueII is a small, electronic device that plugs into your trucks diagnostic port and can read out and display a number of different system parameters. I use one to monitor fuel economy AND as a temp gauge. I can't remember the cost, but it's a lot less than $500.