I have a 1985 f250 6.9 idi with the dual tanks and when I switch to the rear tank diesel just starts pouring everywhere. Not sure if it's a broken line or what, just starting to get into mechanics, thanks for the help

 

Where is it pouring from? Everywhere makes it sound like there is a fountain under the truck....
That makes me think that the return line from the rear tank is broken, but I would trace both lines from the rear tank to the selector valve.

Unless someone installed pumps in the tanks, the way these trucks were plumbed the pump is on the engine and it sucks from whichever tank the selector valve is switched to. This means the feed line is under suction until it hits the pump, and shouldn't leak much since the air would get in easier and stall the engine pretty quick. (a small leak on the feed line would still drip, but not a rapid pouring like you're describing.
The valve also switches the return lines and the fuel gauge wires. The return lines are slightly pressurized with the leftover fuel the engine didn't use, and thus would easily make a mess if there was a crack or hole in the line.

 

^^^ Yeah, need to know where it's coming FROM. One possibility is that when you switch it, the valve switches the supply and not the return. If the tank that's getting the return is full, it will overflow.

Neither tank is "auxiliary", BTW.