The following opinion is for Southwestern Idaho, I don't know anything about Central or Northern Idaho.
I moved here from another state also and noticed the same thing. I was used to designated wheeling areas and signed trails that actually went somewhere. You won't find to much of that here. There are a lot of great wheeling areas and they are working on getting the trails marked and signed but there is still a long way to go. A lot of them are on private property or BLM land. The trails on BLM land are known by the locals and aren't really publicized. Like I said, they are in the process of being marked however I don't think any trail signs have been posted yet. The private property trails are great also, but could be closed down at any time if the owners decided to.
There is a book out now documenting back country drives but I don't think these are really considered four by routes. When you move here, I think you will find plenty of areas to go wheelin, but I agree, there is little or anything posted or documented. I'm just trying to figure out why you would want to leave lovely Houston for Sandpoint Idaho? I lived in Huntsville for about 10yrs, great largemouth fishing around there that's for sure.