Most fast ADCs don't work on 5V and a lot need a differential input, particularly the higher resolution devices. You can reduce the signal level to drive the ADC but you will need to buffer it as most high speed ADCs want a low impedance signal source.
You lose about one bit of resolution by going from 5V to 2V. The effect of noise will be approximately doubled. You'll also need more circuitry to buffer the attenuated input signal like Keith said, which can add more noise/errors.
If all you want to do is compare a signal then a DAC and comparator would be a simpler solution, although without knowing the rest of the design criteria it is impossible to be certain.
You need to pick you ADC - ignoring the 0-5V issue. Then add circuitry to scale the signal. If the ADC is single ended input then a couple of resistors and a buffer might be all that is needed. If it is differential input then you need something like the LTD1992 to provide single ended to differential conversion as well as the scaling.