from general experience, i'd assume 2 options:
1. your airflow IS lower, thus AC "is not cooling as well".
2. your airflow is fine, just AC is not cooling as well.
from option 1, you can check if the air intakes are not clogged; replace EPA filter, as it controls airflow through vents, and when clogged....
the worse option is that your blower starts going boink. either rheostat is dying, or some other electrical thing that controls rpms, or blower motor is dying.
from option 2, simply go to any parts store and get a can of a recharge. you can buy those at walmart for ten bucks with coupling. then just add some "juice" to the system. make sure you buy recharge, not just freon, as recharge has oil in it. adding just freon will, eventually, seize your AC compressor.
to work out option 1, simply find access to the same truck, say - at stealership, and turn the blower on. then compare.
oh, and by the way - wash radiator, esp AC one. sometimes, dust or even insects, can block air flow through it, thus greatly reducing cooling. blow it with compressed air and wash.
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