got a 1999 v8i and it runs nicely does not overheat until i put the aircon on then the temp climbs and it starts to hunt . have changed thermostat and electric fan switch ( they working) aircon pump clutch is not seized i have checked . sometimes after switching aircon off cos of overheating i can hear the sound of running water behind the central dash. it does not seem to loose water and will run for hundreds of km's and no problems until i put aircon on .

 

you have air in your coolant line. look in the HOW TO section in the DISCOVERY section and read how to bleed your coolant line.

 

I agree, need to bleed your coolant system first.
How many miles on your truck, do you know how to test your mechanical fan and have yoiu confirmed the hub clutch is still working OK?