Thanks yammi, I'll look into that and see what comes up. Does anyone else have any more ideas? I replaced the coolant temp sensor and the thermostat (180*) and it didn't help anything. The truck seems to hesitate more when its colder outside.

There's a misprint in my first post. It says that the fuel sending unit has been replaced but it was actually the pump that was replaced back in Oct. 2006. The sending unit stopped working because it isn't compatable with the Airtex pump that was installed. I have no idea if the fuel filter has ever been changed. Could an obstructed fuel system cause my problems? Also the hesitation happens in all gears, neutral, and park. It just seems weird that it only does it on cold days when it's not completely up to temp. Anyone have another idea??