The key is "However once I start driving it cools down."
That means that the system is working as it should as it handles the increased heat that is a buy-product of moving the vehicle, but doesn't handle the "less" heat of idling. What is the difference? Air flow. You have a problem with your fan clutch. (Most likely.)
If you go out to your rig and try and try and rotate the fan (cold w/motor off). If it turns easily, it is bad. (Also If you can not turn the fan of a stone cold engine, the clutch is also bad. but wouldn't cause your overheating symptoms.)
To explain: The fluid inside the clutch needs centrifugal force to distribute and decouple the fan. But the fluid doesn't LOCK the clutch when cold, it should supply some resistance... until spun up. When the air around the clutch gets hot, a little heat sensitive spring on the clutch opens passages inside the clutch causing the fluid to move and couple (or lock up) the fan. If you fan turns easily on a motor that has been off for a while (like overnight) the clutch has most likely lost the fluid.
Note that a decoupled clutch will still make the fan turn, just at a much lower fan speed... not enough to move enough air for cooling. If you turn off a cold (or not hot) motor the fan should free wheel a little before stopping as the clutch should not be coupled. But If you turn off a HOT motor they shouldn't be much (if any) free-wheeling.,,, The clutch should be coupled.
