Hello my name is Dave.
The first place to start would be the spark plugs as they tell a lot about the engine. If they are black then first thing I would check would be the fuel pressure regulator, it is on the fuel rail and has a red plastic vacuum line going to it (assuming it hasn't been replaced), if you hook your vacuum pump up to the regulator it should hold vacuum but before you do that check and make sure there isn't gas coming out of the port on the regulator.
I would recommend running the engine a few minutes before performing the tests, then pull the vacuum line off and see if it's wet and smells of gas. If there is any gas smell the diaphragm has a hole in it and would cause the engine to run rough, especially once warmed up. If it passes these 2 tests I would move on the Idle control valve, it is located just behind the throttle body on the side of the intake and are notorious for causing idle issues with these engines.
Another issue can be the ignition module, on your truck it is located on the driver's fender close to the master cylinder. I would recommend checking with your local parts store and see if they can test it for you, if you can make sure they test it at least 10 times since a lot of the time they only have an issue after they warm up. If they can test it I would go ahead and have it tested to eliminate it as the cause since the testing should be free.
Finally it is possible that the catalytic converter could be getting restricted, this is usually cause by an engine running rich so be sure to check the other issues I have outlined to prevent a recurrence of the problem if that proves to be (part of) the problem.
Please keep us up to date on your progress and thank you for using 2CarPros!
Monday, June 22nd, 2020 AT 4:22 AM