Only certain models are guaranteed as AWD/4WD. That doesn't mean that if you reserve a typical SUV that it won't be AWD/4WD -- just that you're not guaranteed to get one.
Avis and Budget tend to have less AWD/4WD guaranteed models than Enterprise/National/Alamo but the latter usually charges more for them. You could also check for availability of Jeep Wrangler since all of those are guaranteed to have 4WD.
Feel free to enter a quote at autoslash.com and then PM us the inquiry number from the bottom of the quote email you receive and we'd be happy to check for availability of guaranteed AWD/4WD models for your rental.