You are in a similar situation as many cellular data customers.
First up, I would suggest looking somewhere else than EVDO-Coverage, entirely IMO of course. More information:
http://www.evdoforums.com/thread474.html
A dual-band amplifier or an amplified site repeater, either paired with a high gain directional antenna, could potentially help your situation. Note that amplifiers are a grey issue with some carriers, outright banned with other carriers, and questionable with the FCC depending upon actual output. Poor quality cellular ampifliers can quickly disrupt the resources of an otherwise properly working cell tower site.