First off, how is your air flow? If your evaporator is clogged up with dirt and crap it can cause this, a sign would be poor air flow out of the vents.
Second, after checking to see that it is clean, check the condenser behind the grill, make sure it isn't obstructed/full of dirt, bugs, paint, etc.
After you have done those simple checks, now go put some gauges on it, real gauges, not that vhatozone on a can crap with the little green "good" range that doesn't mean jack.
An obstruction in the orifice tube is possible, if this is the case it must be evacuated, replaced, a vacuum pulled and recharged.
Low on refrigerant (hint, it's not Freon) could be a possibility, again, won't know until you put gauges on it, don't just go around recharging it, find the leak first, AC guys will have a leak detector machine, if that doesn't find it they can put dye in the system and look for it. Usually they aren't too difficult to find on these trucks, the o-rings at the compressor block, pressure switches, etc are common places to look.