I think the outside environment is the biggest contributor to the excess humidity right now. That isn't something you'll easily control without the use of a dehumidifier ... which brings us to another point. I'm not entirely sure how well a dehuey will work in a garage. It would have to be pretty air tight or when your electric bill comes, you'll think you were dehumidifying the whole county. The garage may be better than the shed, but you're still fighting a hostile environment.
If it's not been watered since last week and the pot is light/soil feels dry/probes dry, it needs water. Just for now, keep your oscillating fan running 24/7 along with your exhaust system.
Edit: The smaller space of the shed might work better with the dehuey as mentioned by @Ganjadad