my 92 f-150 4.9 extended cab has two tanks. I never use my rear tank because the engine hardly runs when its on that tank. I have a replacement pump but don't know where to start to replace it.

 

Theres 2 ways to do it. either drop the spare tire and lower the tank down. you might wanna use a jack for this or you can remove the bed to get at the tanks from above . I have done it both ways i think removing the bed was easier aslong as you have got some muscle to lift it off good luck

 

CJ is probably right. I've done both tanks by dropping them....and it's not a fun job in November in Northern Minnesota. There is a way to cut a hole in the box to get at the pump and have known many people who have done it that way.
My question is: Are you sure it's the FP? When my pumps went bad my truck didn't run, let alone run poorly. Can you hear pump engage when you turn the key? Did you check pressure at rail? Change filter? Check relays? Bad gas in that tank, especially if you haven't used it regularly? You'd be surprised what gets in a tank. Something could be obstructing the screen on the bottom of pump. (Easy check for that: Drain as much gas out of that tank as you can, refill with good gas and gently bang on tank from bottom with a RUBBER mallet while a buddy turns her over. MIGHT dislodge an obstruction, if present) Check for codes? I'd make SURE it's the FP before replacing it. Plenty of other easier things it might be.