Is there an amplifier such that the gain is equal to the ratio of two potential differences?
There's the AD734 -- not cheap but it should do what you want. (The U input isn't high-impedance; you have to drive it in a feedback loop.)

If you can deal with positive inputs only, and convert them to currents, you can do this with discrete transistors over a fairly wide range. (I tried and failed to find a reference -- basically you have a voltage loop where you take advantage of the logarithmic relationship between current and Vbe of a bipolar transistor)

edit: see Analog Devices appnote MT-079 for some more thoughts