Þórsmörk may seem relatively close to the Ring Road on a map, but you’ll need to take an amphibious bus or go by high-clearance 4WD (super-Jeep tour) to ford the rivers on the way to the reserve (alternatively, you can hike in from Skógar or Landmannalaugar). As you get close, coming from the south, you must cross the dangerous Krossá river. Regular 4WDs cannot make it. You'll see that they are parked where people have hitched rides with buses or super-Jeeps.
Goðaland is the endpoint for the glorious Fimmvörðuháls trek, which starts in Skógar. The main camping area in Goðaland is Básar; to go between it and Þórsmörk by car you must make the dangerous Krossá river crossing mentioned before. Walkers use footbridges – these are on wheels and tend to move around depending on water levels.