Resistance Emulator Circuit Help

[EEV FTW]

I need some suggestion on the easiest way to either buy or build a circuit resistance emulator that output a resistance range of say 58k and 64k ohms. The circuit can't have a fixed resistance so it needs to vary. As far as the frequency of this changing, well faster than 1 hz.

I was thinking of an LM555 timer or something along those lines, but figure someone here new of an easy way to do it with some cheap passive components (resistors, caps, ect). I'd like the circuit to run on say 20v so headroom of maybe 25 volts. I however can always buy a cheap dc buck converter to drop voltage if it helps circuit design.