My truck is runnig hot with the a/c on. It's really bad in stop and go traffic. When I turn the a/c off the temp goes back to normal and stays there. I know that the a/c sits in front of the radiator, but what would cause it not to allow any air to pass through to radiator? Any help would be great.

 

Bugs, dirt, leaves, etc. Any manor of crud. Try cleaning it out with a hose & LIGHT brushing and see what you get.
WC

 

The first two things I would look at would be the coolant and the fan. When the A/C is off there is less strain on the motor so it is easier to cool.
I'm thinking clutch fan, coolant level, water pump.

 

There is no blockage, I checked that already. I have to check my coolant today. Thanks guys