i had the same problem on my 82. turned out to be the fuel rreturn line, as you mentioned. i was able to remove the back 2 cab bolts, and front 2 bed bolts, jack it up a little, and get my hands in there to remove and replace the line. droppingthe fuel tanak is not that hard, and will probably take less time than what i did, but i had a ful tank of gas, and didn't want to siphon it out. good luck