Ok, I'll help you out....
Closest to Nathrop, find Ruby Mtn recreation site (it is a boating access for Browns). County Road 300 takes you to Ruby, and continues through it and up into BLM land. If you follow 300 for a few miles (up 4wd rd) it comes to a sandy draw (Bald Mountain Gulch) there are some random cottonwood groves off this draw that make good campsites.
Next options, go to Buena Vista, drive east (toward the river) on Main St. Take a left just past CKS on Colorado Avenue. This turns into County Road 371. Take 371 north of town for a couple miles until you see a sign for 375 taking off to the right (just before the tunnels). This is Fourmile Recreation Area, it is full of 4wd roads and free camping. There are lots of choices, but my favorites are off 373, 375A and 375C.
These should meet your criteria. Hope you enjoy your trip.
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