1996 GMC Jimmy Won't Start Below 1/2 tank

I have a 1996 GMC Jimmy. It would not start when the fuel level was below 1/2 tank. I put 2.25 Gallons of fuel in the tank, and a little but of fuel down the throttle body, and after a couple attempts, I got it to start. I thought maybe the float was sticking, however when I topped off the gas tank, I was only able to get another 10.82 gallons of fuel into the tank. It used to do this occasionally when it was below 1/4 tank. What could be causing this problem? Also what is the procedure for changing/accessing the fuel pump? Is there an access door under the carpet inside, or do I need to drop the fuel tank? Is there possability that the pickup tube could be cracked, causing it to suck air instead of fuel? If so, is there any way to replace this tube without having to shell out over 200 bucks for a new pump assembly?