Normally, those who have specific requirements shouldn't use Priceline. (That goes for hotel rooms and for cars, too.) Priceline will only guarantee you a "Fullsize SUV," not necessary a high-clearance one. Also, what you consider a full-sized SUV might be different from what Priceline thinks.

FWIW, even if you booked directly with a car rental company, you might not get exactly what you want. And some companies will rent you an SUV but then restrict you from taking it off-road. By booking directly, you can hopefully sort out these issues beforehand.