I live in a condo right next to the freeway. I have an upstairs room that barely receives any air from the central AC unit. I have tried blocking multiple other vents throughout the house to get more flow into the room, and this has worked. However, once summer rolls back around, I will have to uncover these ducts to cool those parts of the condo. This will leave the upstairs bedroom steaming again.

To complicate the situation, the window in that bedroom faces a freeway so a window/wall unit is out of the question since they are not soundproof.

I am out of ideas. Please help!

Sounds like you have an airflow problem. Check filters, Most likely your duckwork is improperly sized, and or, furnace ac unit,is undersized. I believe you will have to call for onsight help on this one. Could be as simple as clogged a-coil, or dirty filter, or clogged condensate coil on furnace(if 90+). To many options for us to really give a good answer. I just really think its all about airflow. Please let me know what you think.