PIC - How to skirt around low maximum ADC input impedance?
The reason the ADC needs a low source impedance is because it has a switched capacitor input.  Basically, whenever the ADC 'samples' the voltage on the pin, a small capacitor is connected, charged up, and then disconnected.  If the impedance is too large, charging the capacitor up will draw enough current to create a voltage drop large enough to affect the reading.  

If you need to read a high speed signal, the best option is to add an amplifier of some sort to provide a low source impedance to the ADC.  However, if you are looking at a relatively slow signal there are a couple of other options.  

One solution to this is to increase the sample time - the length of time the capacitor is connected to the pin.  The chip usually has a limit on how long this time can be, though.  

Alternatively, you can add a decent sized capacitor in parallel with the ADC input pin.  This will decrease the amount of droop that occurs when the ADC sampling capacitor charges up as most of its charge will be drawn from the capacitor instead of through the resistor.