I have a 91 CTD and I pulled the fuel tank so I could replace all of the rusted brake lines.

I found that the fuel supply line had some pinholes from rust and was seeping fuel. Also, the vent on top of the tank appeared to be blocked with dirt. I replaced the fuel line and cleaned the vent and when I drove it for the first time, it was like a new truck. The last time I towed with it before I replaced the brake lines, it felt like it was down on power.

My assumption is that it was either sucking air through the rusted supply line and/or the vent was not allowing enough air into the tank. Might be worth a look.