There are still a couple of companies that sell 6WD (and 8WD) vehicles like these - Argo is one of them (and that machine up-thread looks like they just strapped some stuff to one of those):
https://argoxtv.com/
The other kind of "extreme off-road differential steered" ATV that people both want (and then seemingly want to sell soon after - almost like a boat) is the Russian Sherp:
https://sherpatv.com/
I'd love to have one of those - but they are a bit out of my budget (and my wife would probably divorce me if I brought one home, even if I could afford it).