07 Cummins 6.7l Injectors
 

Stopped at light and accelerated to take off and ran sluggish, knocking and black smoke. Made it home, borrowed an Autel DS708 and checked injectors. Shut off injectors one by one and when #5 shut off no smoke, no knocking and ran smoother. Truck has 254k miles, should I change just that #5? (I am on a budget) Do I have to reprogram anything?

If all you can afford is one that's the method you need to take. But I have a feeling that you fix one and the gremlins will move on to the next one. But that's usually my luck. My truck is covered in gremlin spray at the moment, *fingers crossed*

No programming needed just swap the injector. And keep your fingers crossed as stated above.