there used to be a place within walking distance of the East Gate at CM that rented 4x4.
They did not have any when i wanted one, but plenty of prior advertising & lots of pic on the walls of mainly Suzuki set up for off road use (ie MT and lift to suit)
Also used to be a place in Kanchanaburi area that did proper organised off road trips (fully kitted hilux). Ran by a pom from what i could make out. Ive lost the details, sorry