In Casper there is an off-road park, Poison Spider ORV Area. In fact there is going to be an amateur off-road competition there June 22nd (The Poison Spider Challenge) held every year by a club in Casper.
backcountrycrawlers.com There are a lot of good NF roads and trails around Laramie, the biggest issue you'll run in to is people who forget that their land is legally passable to access public lands.
There is also a book called ""Wyoming Backroads (An Off-Highway Guide to Wyoming's Best Backcountry Drives, 4WD Routes, and ATV Trails)"" by Marc Smith - Open Space Publications - Casper
South and west of Laramie is also the Medicine Bow National Forest that leads in to the Routt National Forest, lots of trails through there also.