boondocking sites in southern Colorado

If you're thinking of taking US 491 from Monticello southeast, you're right, there's not much along that stretch. A couple of possibilities. There's an underutilized CG at McPhee Reservoir near Dolores that has 15 sites (out of 95) with electrical hookups. This isn't a heavily used CG, esp. on weekdays. I think it generally been much less than a quarter full most of the weekdays we have camped there, so a little more privacy. If you don't need hookups, some of the sites are fairly far apart. The nearby Anasazi Heritage Center is a bonus.

If you want a boondocking location, you might check out Google Earth and look for Dolores, CO. Head north out of town on 526/31. If you go north a few miles, you'll see various side roads heading into the brush. For instance here: 37.542116, -108.462735 or here: 37.565129, -108.456461 We've never boondocked there, Allison likes the flush toilets at McPhee ;) But we have mountain biked up in that area and it seems like you should be able to find someplace that meets your needs.

1) Never heard of Monticello Colorado. If you mean Utah, then it is about 100 miles from Monticello to Durango, not five as you stated. 2) Most of the land north of hwy 160 around Durango is USFS, not private. There are numerous CGs and other areas for camping.