Some of the trans-am is suitable for 4 wheeled vehicles...not sure exactly which parts...
For those who do not know, this isn't a singular trail....it is a route consisting of many different roads and trails, all on public land or on public right of way through private land. It was "established" through a bunch of research (reading maps), followed up by exploring those areas to find a suitable route. It tooks something like 10 or 12 years to work it all out....and has been updated a time or two in order to stay accurate (some trails have been closed, some used to go through private land, etc).
The same could be done with a 4x4. I don't know if the endeavor would be 100% successful (the trans am does have some 80+ mile stretches of pavement through the mid west, where suitable dirt roads/trails could not be found).