Anyone know which model year the #8 cylinder problem occurred in?

It was the 6.5L Turbo Deisel engine up to mid 1999. In 1999 they started to install an engine that corrected the problem. There is a good paper on the web that tells all about this problem.

Can't remember where I saw it, maybe www.amghummer.com, or just do a google search for #8 Cylinder problem 6.5L Turbo Deisel. That should find it for you.

The engine cylinder 8 and 1 was never fixed I have replaced many engines well past 1999 year.