You know a suitable voltage source to use could be a simple 10 turn trim pot wired to the boards regulated Vcc +5vdc, just wire the wiper to Aref pin, and adjusted to give you the exact top of range scale you want. A simple life is a happy life.
Well...yes you can do that but there's a catch. You can't control how much current your IC is going to draw so your voltage divider may not exactly work at the voltage you want (I've tried this on another project and instead of getting 5V as calculated, I got about 2.5-3 V).
The best thing to do is to use a regulator or a zener diode rated at 3.3V. I guess since the divider is a pot, then you could simply adjust to get the voltage you want.
Is this the Analog Devices ADXL335 accelerometer? I found that just jumpering the 3.3V pin on the Uno to ARef did the job.