Hello and welcome to 2CarPros. Having owned a few S series trucks, I can tell you that it's probably the fuel pump, especially if it hasn't been replaced yet. If it hasn't been replaced, you've been on borrowed time since about 160,000 miles. The pump has enough to start the truck, then it loses pressure and can't do much more than idle it. What you need to do is go to the local auto parts place with a loan-a-tool program and borrow their fuel pressure test gauge. Under the hood, on the right (driver's side) towards the close to the firewall and nearer the top of the engine, is a valve on the fuel line. That's where you hook your gauge up. Turn the key on and see what it says. Now start the truck and observe the readings. If it's getting down under about 45 psi while idling, you'll have problems. Start there and we'll go from there.
Here's a general how-to guide on checking fuel pressure: https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator
I've attached a picture showing you where the test valve should be.
Thursday, May 2nd, 2019 AT 12:08 PM