Here are pics of (1) the inner fender filler line (2) underneath where the fuel line exits the fender and (3) the line as it snakes around the shock mount to the tank. This is not what I was expecting...I was expecting a separate vent line coming from the tank to allow air to exit the tank as you fill it. Instead, the vent line goes from the filler inlet to the fender well. So this vent line appears to be to allow air to enter/exit the tank when the cap is sealed...which makes sense, both to deal with expansion/contraction of the fuel due to temp changes as well as allowing air to enter as the pump pulls fuel from the tank. But it doesn't help to vent the tank during filling. At all. The vent line is crimped...I can't tell exactly how much when the panel is installed, but I do not feel like it is enough to prevent venting. And anyways, it's not related to my fuel filling problem.
In the 3rd pic, where the line snakes around the shock mount, it does dip a bit...nearly with width of the line. However, I'm not feeling remotely confident that this is a problem. I don't see why fuel blocking the inlet here would have any different effect at 1/4 tank than it would at near-full conditions. I'm guessing that the rubber hose with clamps did not come that way from the factory...indicating that this line was (partially) replaced at one point. There appears to be a part of a clamp that looked like it would have held the line in place near the shock mount. But it looks like it was cut and not replaced.
Maybe I _am_ filling the tank completely and either the gauge or the sending unit is off. Is the sending unit the same as an 87 GW? I have a new-in-box replacement. I suppose I should test the currently installed unit. And/or the gauge.
I should also try filling with the wagon parked facing downhill to see if anything is different. I know a place where I can do that...this should allow me to completely fill the tank and then I can see what the gauge is indicating.
77 Wagoneer and 97 TJ.