My initial guess would be a decent vacuum leak or vacuum line plugged up somewhere too. Those can cause some very strange symptoms for sure. When you rev it up does it rev normally or is there a noticeable lag when you first press down the gas pedal ?
Is the A/C on (if it has it) when this happens ? Also, with dual tanks, isn't there some kind of check valve between them ? I don't know for sure though as I have no experience with Fords around that year but Mustangs. I did have a problem with an 88 Mustang GT where it would either do what you're saying or would surge up and down when in P or N. It turned out, on it, to be the Idle Air Control (IAC) valve.
As I said, no experience with Ford trucks from that time frame but IF it has a throttle body or MAF sensor have you looked inside them to make sure they're not all gunked up ?