Are you sure the stock speakers are 8 ohms? I thought I remembered fluking them at 4 ohms, but if you're reading 8 maybe try some Pro Audio speakers? Something from Parts Express, I've heard the Selenium 6" speakers in person and they sound great.
Keep in mind, that audio systems are measured in terms of impedance, not resistance.
A Fluke multi-meter will give the dc resistance of the coil windings. Impedance is the ac resistance offered by the coil winding while under power. This value varies, as the frequency of the ac voltage changes.
Impedance and resistance share ohms law results, so that when speakers are connected in series, the impedance values are added, as well, when in parallel, the values are reduced.
Generally, vehicle audio systems (because we only have a 12 volt supply) use 4 or 2 ohm amps and speakers. The thought being, that low load equals high volume.
The difference between front and rear may be a result of speakers being in series, or in parallel.