81 GMC 1/2 ton... rough idle when warmed up

Chexk everything and then recheck. I have seen brand new caps bad right out of the box. Also check for any danged pkuga. I've Seen Those CRACKED New Out THE Box.

Check possible ignition coil arcing to ground or possible distributor pickup coil or module.

Definitely the plugs too. If the wires are many years old, good to change them. If they are the original wires, definitely change them.
Also new gas filter, and new air filter.

TUNE-UP, SPARK PLUGS, WIRES, AIR & FUEL FILTERS, FUEL INJECTOR CLEANER IN TANK, ALSO REPLACE PCV VALVE, & MOST IMPORTANT, REPLACE EGR VALVE, IT GETS TOTALLY CLOGED UP, YOUR ENGINE IS KNOWN FOR THAT. I HAVE A GNC 2500, 3/4 TON. SEEN IT ALL. CHANGE YOUR DISTRIBUTOR CAP & ROTOR ALSO. JUST DO IT !!!!!!!!!!!

check to see if your egr vave is sticking. if so you will need to replace it.

I AM LOOKING AT TIMING ISSUES OR INTAKE MANIFOLD LEAK.OR EGR VALVE STUCK OPEN.

timing might be off a little...or your low idle control might not be functioning. try unhooking your idle control module on throttle body, if it doesnt change idle when you disconnect it, its faulty.

Try a new distributer cap and rotor button,these are notorious for wearing out the carbon pick-ups and leaving carbon tracks in the cap which messes up the fireing order.If that does'nt fix it try plugs and wires.

try the plugs plug wires and cap and rotor

My problem was solved when I repaired my 4 wd activation dash switch. The switch was contacting on 2WD & 4HI at the same time. The front differential activation is vacuum activated possibly creating a tug of war that didn't actually engage it but drained off vacuum from the engine? A theory at best but stumbbled across it because I knew the switch wasn't working properly I repaired it and low and behold rough idle disappeared and all is good for 3 weeks running.