Not Boondocking, but nice places to overnight:
We've gone by Thermopolis, WY twice to do a soak in the hot spring water at the State Bath House. Very neat.
We overnighted in the Wind River Canyon in Aug '09 & at one of the Boysen State Park campgrounds by the lake some years ago.
Hot SpringsWe did an overnight, years ago, in a forest service campground not far from the East Entrance to Yellowstone via Cody as well.
Overnighting Sites & Sights in the
Douglas & Wheatland Area. Includes Guernsey & Fort Laramie Area.
Guernsey State Park -------
Fort Laramie National Historic Site ------
Oregon Trail Ruts and
Register Cliff Find a National Forest were you want to camp, then look up the information to find the sites in them, such as:
Big Horn NF Camping.