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Hire the car you need, and pay the cost.

Every rental car agreement I've ever seen says specifically "this vehicle is for public roads only"  and that has implications to the insurance.  

If you get in trouble off-road then your insurance will be null and void, plus you'll be liable for all recovery and repair, out of pocket.

Hire a 4WD with a off-road rider in the paperwork, don't stick a pocket torch to your rental camry and get stuck in a hole.