@PenmanPhoto
My experience in Vegas/Death Valley was that the contracts from all major carriers said you could not take the car down a dirt road let alone off road.
If you want a full blown Overland rig. Call or Email John at Tonto Trails out of Durango (about 6 hours form Denver in the beautiful SW corner of Colorado)
info@tontotrails.com 970.403.5865 John is an experienced Overlander who has been helping me plan my next trip. He rents out well equipt Sportsmobile Overland vans.
Closer to Denver:
Green Jeep Tours out of Estes Park may also rent Jeeps but I think they are just a tour service. Friends have taken their Photo Safari Tour and were very happy with the trip.
Searching I found this: Looks like an airbnb for car rentals!
https://turo.com/rentals/suvs/co/estes-park/jeep-cherokee/26146 Not sure if you're into the whole sharing economy but it's an option.
If you can catch a shuttle to Breckenridge Co (~2hours from Denver on I70) try
http://mountainhighrentals.com/ . They rent off road Jeeps. I have not personally used them but I was talking to another guest when I was staying up there a year or so ago and they used them every time they came to Brek.
I think "Rocky mountain jeep rentals" is out of South Denver or Castle Rock Area I see their trucks on the road and trails in the summer. I think they rent out Wranglers and the Larger Jeep SUVs.
Boort