[93GTDIN]

I just had the exact same problem. FMU's are tricky pieces of equipment that must be calibrated to your situation. On my 93 GT, when I hit it on the highway, from 0 - 3 PSI it hesitated and sometimes shook. I also had thick smoke that reeked of gas coming out of my tailpipes. I was able to deduce that my FMU was jacking up the pressure to early. The way I fixed it was by unpluging the FMU. My car, at stock pressure, had enough fuel to not run lean. Your truck may be different, so i would try the following things:
Make sure the vacuum line from the FMU to the tree has a check valve and more importantly, a vacuum restrictor. This will stop the FMU from putting the fuel system under pressure prematurely. Also, try opening your FMU (turn left) in 1/4 turn incraments. You should hit a sort of "sweet spot" that will work best for you. If you open it up all the way and the hesitation is still there, 4:1 might be to much for your situation. At this point, i would try simply unplugging the vacuum line to the FMU (No need to unplug the fuel lines) and give it a run. It may ping, so be careful. I would highly recommend an a/f gauge for your setup.

hope this helps
dino