Probably just a defective ring. It happens. I had a ring break in a low mileage 6 cyl Jeep this summer.

How is your oil pressure? Is it lower than normal? What does the spark plug look like in that cylinder? Is it peened.

It might be possible to just drop the pan, pull that piston, hone the cylinder, put a new set of rings on that piston and put er back together. Did that on a Mercedes. They are not supposed to crater either, but this one had only 117k. ( not mine)

If the compressions and oil consumption on the other cylinders are okay- no need to do a rebuild.
I can't remember what is under the pan on that truck, if you have acess to the pan.

If not, pulling the motor wil just add a few hours labor, to get to it.