What causes the engine to shut off while driving?

Fuel filter may be dirty if it starts up almost immediately then it may be a really dirty tank causing the filter sock to clog. A filter generally takes anywhere from 10 mins to an hr before restartup. A dirty fuel filter sock can drop the crap almost as soon as the pump shuts off allowing an immediate restart. Hook up a fuel pressure gauge and drive it around with the guage visible from inside. I'm guessing it has a 4.3L TBI in it? The pressure should NEVER drop below 9psi it should stay between 10-13 psi.

If no service engine soon light comes on, then like our 92 Astro AWD with mystery stall, the problem was the fuel injection system (Spyder)... you have to replace the whole unit. It's located in the upper intake mannifold and took me @ 4 hrs to replace.

You could also check catalyst converters

I just replaced a coil on a 92 that would stall and not start until it cooled down and that fixed the problem