No power at fuel pump.

The video may help?

https://www.google.com/search?q=1999+GMC+Sierra+Classic+3500+4x4+shift+down+while+driving+down+the+road

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check fuel pressure should be 42 lbs. check filter if low some engines have 2 filters

usually a sudden stop is sensor gone out if it fuelor timing it would have erratic or skip tostart with i would put it on a diagnostic tester to be sure

If wires came off oil pressure switch or if switch went bad, it would shut down truck immediately. Your oil pressure switch performs 2 functions; oil pressure to gauge.light and it's also a safety feature that if you're in an accident or rollover, when oil pressure drops it shuts off power to fuel pump. I'd start there. The reverse is also a possibility, if fuel pump fails, truck shuts down = no oil pressure.

If its trying to start it could be the fuel pump. Try some start fluid.You should hear the fuel pump cut on when you turn on the ignition for about 2 seconds. If you have fuel check to make sure you have fire. Take a plug wire off and put a screwdriver in it and hold against something metal while turning the engine over. You may need help to do this.If you have fire then its timing. Could be the cashaft position sensor.

Sounds like the fuel pump. Simple check put key in ignition turn to on position Pump should buzz. Another way to check use a rag and hunt down schrater valve on the fuel rail on engine. Wear eye protection put rag around bottom of schrater valve and press down on valve. Fuel should shoot up at you if not your pump is bad.

check fuel pump relay, is fuel present at motor 

could be as easy as a clogged fuel filter or as bad as putting in a new fuel pump

sound like you are not getting the right fuel pressure you need to check the fuel pressure hit the tank in the center with a hammer and se if it runs if it does you need a pump

Sounds like the fuel supply is inadequate. Check for a plugged fuel filter, or possibly the fuel pump is losing pressure. I had a 93 Chevy V6 that acted the same way, and after checking everything else I replaced the fuel pump, which solved the problem.