I guess first question is............is engine overheating?
Is overflow tank full or bubbling?
1999 Ranger will have 4.0l OHV, and a GEM module for "body" electrics
It has 2 temp sensing devices on the front lower intake of the engine
A 2 wire ECT sensor, used only by the computer
A 2 wire Temp Sender, used only by the dash board gauge
The 2 wire Sender is the one you need to test, it will have Red/white stripe wire and a Black/white stripe wire.
Remove it's connector
Get a Jumper wire and connect it to the Red/white stripe wire by pushing it into the connector
Turn on the key, engine off
Temp gauge should be below C on the gauge since wire is unhooked
Ground the other end of the jumper wire to the engine, or battery negative
Gauge should move up all the way to H or above
If both of these things happen then replace Sender, make sure you get a SENDER not ECT sensor.
What years were the explorer dash boards, and do the other gauges seem to work OK?