I just replaced my rear speakers in my '99 SLT. It has the infinity system. The rear speakers are indeed 2 ohm impedance. Not sure about the fronts. The amp will perform best with the same load. Larger loads (4 ohm speakers) will not play as efficiently as 2 ohm speakers.
I bought a pair of Kappa 62.7 speakers (consider eBay), but I had to do some modification to make them fit (Kappas have 4 mounting holes, stock infinity have 3). I found out afterward that adapter rings are available for about $10.
Somewhat off topic: the speakers are technically not "blown". Blown refers to melting through the wire in the voice coil. This results in no sound reproduction from the speakers.
More than likely, your rubber surrounds on the speakers deteriorated from the harsh environment (sun light, temperature, and moisture). The end result is sound reproduction of mids and highs with distortion of the bass (which sounds like what you're experiencing). You can re-cone the speakers (
www.partsexpress.com probably has the parts), but who wants to deal with that...