AC/Heater Fan Blows Poorly At The Vent

My '98 4x4 Chevy PU truck has poor air flow at the vents as though the ducts are obstructed. I'm thinking leaves. I can hear what I suppose are internal
doors of some sort opening and closing when I adjust/redirect the air flow and the fan seems to be doing it's job. Is there a way to clean out the ducts short of taking them apart? Do I risk damage if I blow back with a leaf blower to try to blow out any obstruction? David
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