One of the motors has dirt/mud grit inside the housing, and now won't turn as freely. I quickly turned off the drone and haven't turned it back on since.
Can I take apart the housing to clean it out without voiding the DJI Care Refresh warranty?

(If I can clean it out and get back in the air, I'd prefer to do that; once I clean it out, I'm going to look into how to do prop balancing before I consider sending it in for work.)
Also, this occured because it crashed at launch and I need to figure out why that happened; I launched it by hand, but I could immediately feel the right side motors pulling up more than the left side. Visual positioning was off (shooting over water and hand catching), but I saw GPS mode before I throttled up so I figured it was safe to launch. It drifted hard left into the leaves of a tree and then landed at the edge of the river with one leg in the mud.

I think the rest of it stayed dry but I'm letting it air out for a few days just in case. I had just flown it successfully a few minutes earlier with no issues.

If you attempt any repair yourself you will void your warranty. Even if you attempt to clean it yourself, if you are not familiar with electric rc motors then you should not attempt it. Considering it crashed and the motor is full of dirt it may have bent a shaft.
The choice is yours but if you don't successfully fix it yourself then you stand the chance of the motor failing in flight which could seriously damage the Mavic.

Jamie, we are sorry to hear about the accident.
Personally, I would suggest you to send it in to DJI service center, our engineers will help you assess the damage and get it fixed for you.
As mentioned above, damage caused by unauthorized repair will void the warranty.