engine will not idle
Sort of like telling us the truck is blue and expecting us to diagnose a problem.
Tell us about your truck - what engine, transmission, mileage, ect
Tells us about your problem, all about it. Won’t idle… Ok, badly? Stalls? When hot? when cold? All the time? What?
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Well, what kind of Chevy 1500 is this?
Express, Silverado, etc.?
What engine?
4.3 or 5.7?
Since you gave us next to nothing, I’ll just throw something out there
idle air control valve
A 2000 will have either a 4.3 or an ls motor (4.8, 5.3, 6.0). Easiest thing to do first is clean the throttle body. It probably won’t be that simple unless you’re lucky, but it may help the idle.
replace all parts associated with the idle function. might work. 
Not if this mystery vehicle is a Chevy 1500 Express
In that case, a 1500 Express might very well have a 5.7 liter V8 . . . definitely not an LS motor
Well you’re right. The mystery vehicle turned into a Silverado in my head.
In recognition of the sphinx-like question, my entire, reasoned answer is the “one size fits all” - vacuum leak.