[Jeremy-WI]

There may be a hose going from the top of your fuel tank to the filler neck, its supposed to vent air from the tank to the outside while filling, sounds like yours may be plugged. If it doesn't have the hose like I described it could be a problem with the EVAP system. Send a PM to BlazerLT - he posts regularly on the Chevy Blazer and GMC Jimmy forums and should be more familiar with the system used