It could be the the tank selector valve mounted down on the frame under the drivers door. Or it could be a problem with your rear pickup tube, or the line from the tank to the switching valve. If the "showerhead" in the rear tank has failed and a piece is blocking or stuck in the end of the pickup tube or it's full of alge/slime, then it won't flow the fuel required. Since you describe the problem as cutting out/surging when the tank is full, I would suspect the valve or the line to the rear tank first. But since you also describe the filter light coming on, that only happens when the injector pump pulls a vacuum on the filter header which means the lift pump can't deliver fuel through the filter to keep it happy. The valves can be had on ebay now for ~$50 but I would check that rear line for cracks or breaks or a plugged up rear pickup before I blew more money on it. Maybe you can pull the rear feed line at the valve and apply a low pressure to the rear filler and see if fuel flows out easily? Do the same with the front to compare flows?
