gotcha! i was thinking you possibly meant to insure your rig just against damage you do to it on the trails, and was wondering what insurance company in their right mind would ever offer that (without charging an arm and a leg). but if it's just for liability, that makes sense now
I had a Polaris RZR insured with full coverage for $400.00/yr, if I tumbled it off a cliff they would have covered it.
OP, keep in mind if you insure it for off-road only and are caught driving on road, like between the staging areas or down the dirt road at Maclean, you will be eligible for the $2,500 fine.
Best thing to do is call an insurance company and explain exactly what you want, minimal insurance that will cover you on road and off. Explain that the only on road driving will be between trail heads.