I had the same thing happen with my 1993 Dodge D350. Fuel was getting in the oil pan, so I switched over to a low pressure piston pump, and again I still had the same problem.
I then pulled all the injectors and they all seemed to be fine, so the injector pump was the next possibility. I pulled it next ( what a pain in the butt, its no easy task), but as it turned out the front seal was leaking. I want to get the tools and learn how to reseal a pump myself. The diesel shop's have a tendency to want to big deal everything, even when its something simple.

On the note of a lift pump, do the p pump trucks have a " low pressure " lift pump and a "high" pressure lift pump? Or what's the growth on the side of the piston pumps?