Year? Engine?
If we ASS_U_ME it is an 88, and maybe a 4.0, then the temp gauge sensor is in the head near the firewall, driver's side.
It is a single wire sensor, so ground the connector and the gauge should max out with the key on. If it doesn't, then test that wire from the connector all the way to the instrument cluster--if it is good, then the cluster is bad.
Note that there are TWO sending units--one for an idiot light and one for a gauge. The idiot light sender is an on/off type, the gauge sender is a variable resistance unit.