300 SD Fuel Tank
 

Is there anything in a 1984 300SD Fuel tank that can restrict the fuel return line and cause back pressure?

Don't think so. Have you tried blowing back thru the return line?

Yes. Some suggested it could be the vent valve. I do get a whoosh when I open the tank.

A plugged vent wouldn't cause backpressure in the return line. It would cause increased suction on the return line. But if you are getting a whoosh you might have a plugged vent.
There is another possibility--someone may have switched the input and return lines. If that is the case, then a plugged tank screen at the end of the return line might inhibit return flow.

Evidently, "some" got a better description of the situation than has been provided here. What is the rest of the story?

Unless thermal expansion is a factor.

Same description as here.

I guess my symptoms should go away if I leave my filler cap cracked open as a test.

IF it's tank vent related...