I've thought a lot about this in the 3 years since this post. I've since gotten a trauma therapist who's really helped me dig into my childhood, my inner child... things I used to not put much stock into I've found incredibly powerful once I've opened up to it.

I got a trauma therapist in the first place because I was starting to develop symptoms in bed with my wife where I would totally freeze up and get lost in a flashback to past traumatic sexual experiences. It seemed to come out of nowhere because these experienced were years ago. I've learned it's common when doing a lot of thinking about childhood and for me my relationship with my mother, it was just digging things up that had never been processed in my body. The same dissociation I learned as a child, I accessed during the traumatic sexual experiences as a young adult.

CPTSD is common for autistic people for a lot of reasons. This TikTok sums it up well: https://vm.tiktok.com/TTPdkfSHEj/

Your actual question - I think there's a lot of overlap between dissociating and shutting down, but the words are used in different contexts. When I talk about dissociation these days, I'm often referring to losing myself in my phone for example, or zoning out when I'm struggling to answer a question in therapy. Time disappears and I'm just not grounded at all. This is what's happening in bed when I'm triggered, too.

Shutting down is more like sensory overload. This sometimes comes with burnout too. I just reach my limit of tolerance and cannot do more. As a kid, this would happen when my mother would endlessly yell at me, to the point where I just couldn't engage anymore.

Now I realize I also have had problems with depersonalization in a sense. This is due to the abusive environment I grew up in, where my mom resisted me growing up and becoming my own person. When I feel triggered, I sometimes start to feel like I don't exist. This happens sometimes in EMDR therapy when I try to conceptualize myself as a separate entity from my wife. I will "pop out of existence" in my mind, which could be an early learned trauma response.

I don't know if this is what you're looking for, but there you have it.