I may be just being lazy about this, but my truck does the same thing. If I fill both tanks and run on the rear one, it overfills the front one. If I run on the front one, it runs out normally. Self curing problem in my book. I run on the front one until it is low, then switch to the rear. After it is down to 1/8th a tank, I run the front again. Actually, my front tank doesn't like to be filled from the neck, for some unknown reason, so I typically just fill the rear one. I can fill it twice before switching to the front one. MPG is curious to determine this way, but my truck likes gas too much to want to know just how bad it is anyway. With 32's on, my speedo is off some too. At best guess, I think I am getting around 14mpg, so it is within normal ranges.
If time or money is tight, I'd try this rotation method until you find enough motivation or other reasons to start digging into the system. If not, then there you go. I'd follow Sean's advice and change out that part.
'90 F150 300-6 m5od 4x long bed