Take 20/26 west to Shoshoni and follow 20 north through the Wind River Canyon to Thermopolis. From there take 120 towards Cody and then take 14/16/20 through Cody and into Yellowstone. The roads are all well maintained 2 lane with no climbs of any real worry until you make the climb into Yellowstone. This is the East entrance and will take you directly to Fishing Bridge. The climb into YNP is a little steep and winding, but just take your time and you should have no problems.
This is the route we use.
Keith
