A big issue with PTSD in troops was frequent combat rotations. My understanding of dane vikings is they raided frequently. But is there any documentation of what they felt after?
Not to discourage any other answers, none of these deal with vikings, but pre-modern PTSD is a rather common question on this sub, so there are quite a few good answers if you look. I'll just link u/hillsonghoods Monday Methods post on the subject.
I'd also point to this post by u/sunagainstgold which touches on why this question might not be the best one to ask in some ways.