Yeah the one on the beach is the sand king XL I believe. A very large pick up truck and while the tires are big, you won't be running over cars with them. Not sure why they never put a full size monster truck in a GTA game. Saints Row and Just Cause 2 had them though.
Also, there is this MASSIVE dump truck type vehicle at the quarry which is off to the right of the map somewhere. It's called Davis Quartz or something like that.
It's really slow but those tires are ridiculously large. Closest thing to a monster truck you'll find.