I2C Pulldown and Pullup...is it possible?
You're right that a pull-up and a pull-down connected to the same line would create a voltage divider.  

You could connect the pull- resistors to a digital I/O pin.  If you drive this auxiliary pin high, it will be a pull-up.  If you drive it low, if will be a pull-down.  If you let it float, you can make an undisturbed analog measurement.

You could also use a larger PIC with more I/O, such as PIC18F4550.  Then you wouldn't have to multi-purpose the pins as much.