Debugging my bipolar stepper motor design
Okay, you have a couple of issues:


Your voltage divider will present ~2.55V at it's centre, but only unloaded - due to Ohm's law as soon as you present a low resistance load the voltage will drop severely. For instance, if your stepper coil has a resistance of around 3 Ohms, then the voltage will drop to ~9mV!. See the various wiki pages on maximum power transfer and voltage dividers.
Rather than making sure you have exactly the right voltage, you would be better off with a stepper motor driver that will ensure no more power than the motor can handle will be used. They do this by "chopping" the voltage into pulses, and there are many to choose from. The L298 is pretty old now, uses bipolar transistors and after the source/sink saturation voltages you will have almost nothing left to drive the motor. I would read this wiki page on Stepper Driving (and related pages) plus ask people who have already used these setups - the reprap folks are probably a good source of info.


On dropping small amounts of voltage, a series diode can come in handy to drop between 0.3 and 0.8V or so per diode. But for efficient power regulation, depending on the difference between input and output, linear or switching regulators are the norm, (almost) never voltage dividers for supplying power.