Do I need a 48V switch for a voltage divider circuit?
In the current configuration, and in fact any series configuraiton, when the switch is open, you'll have the full 48V across it -- no current flows through R1 or R2, so each drops no voltage, leaving the full drop across the switch.
If you don't mind wasting a small amount of power (not good for batteries, but OK for DC supply), then you could alter the topology to allow a switch for a lower rated voltage. By placing the switch in parallel with R1, you would still have the same 48V/36V gate drive depending on the switch position, and the maximum voltage seen by the switch will be the 12V -- this works because R2 is now always conducting a current and so always dropping some of the voltage. The switch of course has reverse meaning - closed = off, open = on.