Free public land shooting: your best bet is probably Pawnee Grasslands, about an hour north and a little east of Greeley. Everyone has their favorite spots up there. Be picky about the times when you go (avoid weekends). Be courteous and pack out more trash than you bring in.

Other than that, you may want to contact RW Swainson either through Facebook or you could PM Buckeroobonsai and see if he will take you out and give you an introduction. RW's Bennett farm are not free, but for the variety of shooting you can do there, the price and atmosphere are well worth it.

This isn't Utah. Lots of people actually live here in the supposedly uninhabited plains and mountains. I know that if you go just west of Tooele, you could shoot for miles without hitting anything other than sand or rocks.

Be safe.