I've found my '93 filler tube to be quite temperamental, but it works
fine if you work with it.  I usually try to pay attention to how much
gas has gone in, the I slow the pump down for the last gallon or so.
Also, try angling the gas pump nozzle down into the filler tube.
And keep a rag handy for cleaning up the spillage.

It sounds like your '91 might have the "15" gallon tank. If this is the
case what you have is a 20 gallon tank with a plastic hose inserted down
5 gallons into the tank which leaves a large air space above which can
blow back when filling. The hose can be pulled out most of the way to
convert to 20 gal and eliminate the blow back.

I had an old car that had the same problem. When you think the tank is
nearly full, slow down the filler to the very slow untill it stops.

Like Jeff mentioned, there is a mod to increase the tank capacity and
eliminate the blowback problem.

HTH

Carl