I actually heard they got most of the show either in the can or with produced scripts since the episode order was smaller than most shows. I think they only got 13 or 15 ordered for the entire season instead of the normal 22, regardless of the strike.
The last episode made no sense because of the Writers strike. Kelso retired and Dr. Cox took over as Chief. Then in the last episode Kelso was back with no explanation. I read that they felt that episode was the best episode they had in the book for a finale so they showed the final episodes out of sequence. It was lame.
Hopefully, they tidy it up next season to explain that Kelso was only back to help train Dr. Cox or something.