When renting Jeeps from a car rental company in Cozumel, they require that you do NOT go off road in their jeeps. If you want the off-road experience or the Dune Buggy tours, make sure you book that with a company who has the appropriate vehicles for this type of driving. You can do it through the cruiseline, or if you do a search online, there are other independent companies that come highly recommended.
We do rent Jeeps everytime we are in Cozumel, its a great way to see the island, you get so much more freedom and better time management this way. But for the off-road stuff, stick with a tour company.