I'd recommend a 555 timer - there are circuits online that do this. They're known rather humorously as 'servo exercisers'. They work by having one 555 generate a pulse train at the required frequency (usually around 50 Hz) and sending it to a second 555 in monostable multivibrator mode that controls the duty cycle. By changing the RC set circuit on the second 555, you can make the servo move back and forth. Find a combination that gets you the two positions you want (most likely by having two different resistors) and use a switch to choose between which one connects to the 555.
Here's a schematic from the good people at the Seattle Robotics Society:
http://www.seattlerobotics.org/encod...Excerciser.htm
Good luck!
Sparks