Answering my own question:
After some digging, I found out my mistake.
The jumper goes on the 3-pin DIP header at the end of the Analog Breakout, not on the input pins for Input 8. I'll have to try it when I get back to the school to confirm, but I'm pretty certain this will solve my problem. Well at least THAT problem.
I should have just walked over to last year's robot and looked at it!
FRC Control System Data Sheets (Page 11)