GMC Sierra

How long since a tune up? Any applicable trouble codes? The basics for a rough idle, check for a leak of some kind, fuel egr vacuum. Just about anything can cause a misfire, fuel spark compression. if you think it is spark, you can use digital multimeter on low voltage setting, engine idling, touch the spark plug wires one at a time, I've seen readings like .6 or .7 volts, if readings bouncing off zero, there is your problem, whether spark plug wire or whatever? You don't take plug wire loose or penetrate insulation, It's an inductive voltage.

I've got a guy looking at it for me and he thinks it's a fuel problem, spider injection maybe. All cylinders and plugs are good he says. Just don't understand because it was running fine when it died

good luck