I found the solution for me. I tried a ton of different things with the drivers (plus other things), but the problem was simply I wasn't logging in right. Weird, I know.
So when I connected to my DB2 data server via Cognos Analytics (Manage -> Data servers), I was using the second radio button 'Prompt for Username and Password'. I tested the connection and it succeeded, so I just pressed 'OK' and figured I was connected as I should be (it showed that I was connected to the DB Server and everything.)
Well, turns out, you really have to connect using the fourth radio button 'Use the following saved credentials' and then enter in your Username and Password for your DB.
I know this is sort of a dumb mistake, and it probably isn't the problem you're having, but I thought I'd let you know. I never thought to go back to step 1 (logging in) when everything seemed to have gone so smoothly at this step.
Hopefully that helps you,