Sounds like you are low on refrigerant but also could be several items. Do you have gauges to check the pressure, start there. Also, if you are Odessa, FL you might want to make sure you are not having a/c problems due to the love bugs jammed into the cooling system. Are you sure the cooling fan is engaging, these trucks in that year used an engine driven cooling fan with a electronic clutch, if it goes bad it won't pull air through the condenser and thus no cool air inside.
For what it's worth when I had the SAME issue on my 08 I was low on refrigerant, just a bit but our A/C tech at the shop vac'ed my system down and added about .25 lbs more than required. That really helped things and I was never complaining about cool air again.
if it is worse at idle(therefore better when you are cruising), chances are you are not removing enough heat from the condenser. possibly could be cooling fan issue or your condenser is plugged. if you want you can try spraying cold water on the condenser with the vehicle running, if the vent temp drops, it is more than likely one or both of these two problems. FYI adding too much refrigerant can cause vent temp to be warmer.