Hi there,
Im not entirely sure i understand your circuit, but it sounds like you are using a constant current source to power a resistor, then measuring the voltage with the ADC, then calculating the resistance using the uC code. It also sounds like your constant current source with some resistor values can put out as much as 17vdc, and that is too high for the ADC input so you want to scale it to 5vdc.
If this is all correct, then my question is why cant you lower the constant current source current level so that the higher value resistors still come in less than or equal to 5vdc, which would be perfect for the ADC input?
For example, if you want to measure as high up as a 5000 ohm resistor then make the constant current source put out 1ma, as that will give you roughly 5v for that resistor value. Of course as the resistor value comes down the accuracy will start to suffer, so you may want to switch to a 10ma current for smaller resistor values through program code and a variable constant current source.