1999 Chevy Blazer leaking gas

I have a 99 chevy blazer in the last year I got a new trasmission. In June the fuel pump went bad so we changed it. It had been going bad for a long time. The fuel gage hadnt worked in years. But since we changed I can not fill the car FULL with gas. If I do with in 15 miles of driving gas starts pouring out of the back end (like where the spare tire is) and the car stalls out. If I let it set for an hour or 2 and the gas leak stops I can drive it just fine. Well if the gas gets below 1/4 of a tank it stalls every time I stop. We change the fuel filter and the canistere vap valve. I dont know what to do next. It is also very very rough starting sometime can take 10 to 15 seconds before it will finally turn over and start