My van is a JDM 1991 Toyota Townace with 68k miles, so its an unusual vehicle for my mechanic to fix, he's used to repairing newer domestic vehicles. I need some help on trouble shooting this failure in the fuel system, and don't know how to properly navigate diagnosing this issue... Looking for ideas to rely to my mechanic.

So the other week I was traveling on the highway with the van, as I was heading down a slight gradual slope, the van suddenly took off with extra power and increased acceleration. Luckily I was not in danger of crashing (straight road, had plenty of distance from cars), but after I removed my foot from the pedal, the van continued in to accelerate. At this moment I became freaked out, but placed the transmission into neutral. This obviously caused the engine to rev at a high degree, and I cannot remember if I had immediately turned the ignition off or the van shut off on its own. But there I was, coasting to the shoulder of the highway, from there I was towed to the mechanic. In hindsight, about 72 hours prior to this sudden acceleration event, I noticed sluggish ascents on hills, which may have been the precursor to the eventual breakdown.
After my mechanic's initial review, all the fuel was drain and replaced (he thought he smelled gas, and implied I may have filled the tank with gas, but the diesel is a prepay pump, anyways, we started here). Now, the van idles fine, however as soon as you engage the gas pedal, there is no power, and if I remember correctly, the engine floods and wants to stall as a result. His suggestion is to send the fuel pump to a specialist for a rebuild.

How common or rare is it for fuel pumps to fail with this engine? My friends are suspicious that this is not the real problem...

Has anyone experience something like this in there vehicle?

Can anyone help by providing a list of suggestions that may resolve this systemic failure, starting with the cheapest/easiest solutions to... rebuilding the fuel pump?

Any guidance is greatly appreciated, feel stuck at the moment and need resolutions!